A valuable British mini-computer is stolen from its factory by foreign agents. It is a prized piece of British technology, now in the hands of the Dreisenbergian Ambassador who plans to smuggle it out of England under the cloak of diplomatic immunity. In steps the bishop who sends our heroes, Father Unwin and Matthew, on their trail before the Dreisenberg airliner, Helga, takes off. Arriving at Heathrow, Unwin is able to get Matthew, now minimised for his mission, close enough to the hangar of the 'plane. Here he sneaks aboard the unattended open propeller engine to await the take-off. Once in flight the Ambassador and his aide gloat over their success in getting this far with the theft but they don't expect Matthew, under Father Unwin's instructions, to stop half of the 'plane's propeller's, forcing it to land at a disused airfield. At Oakington Father Unwin speeds to Matthew's aid but is flagged down by police... for speeding! Using the gift of Unwinese double-talk, he convinces the police to forget the speeding and head with him to the airfield. Father Unwin is able to get the mini-computer out from the Ambassador's clutches using the Minimiser but, as the police arrive, the Ambassador seizes control of their car and uses it to pursue Unwin who has a good lead by now. The Father's local knowledge enables him and Matthew to out-manoeuvre the Ambassador and his aide, to bring the computer back to safety. Keeping with his other calling, Unwin is able to use the mission as the focal point of his next sermon.

At 250 hours of operation, a British-designed desalination plant is sabotaged by explosions so the Bishop and Father Unwin are called in, mindful that added problems at other such world-wide plants could scare off potential customers, mainly a big U.S. company. Additional security at the next plant, Burgossa, fails to prevent yet another similar attack so now it's up to the Bishop to prevent the local British plant, Port Trennick, from the same fate. Agent Blake is called upon to act as an 'inside man'. He will take a knock-out pill to coincide with Father Unwin's arrival at the main gate. The Father is to perform the Last Rites and, at the same time, aid Matthew (who by this time will be underwater) to find out the method of attack. It turns out that the sabotage centres around the water intake pipe. Frogmen blow an opening in the mesh screen over the opening, then return with a small timed torpedo, deploy it and return to a nearby submarine. Matthew is a spectator at the latest attempt but, after the screen is blown, stretches a screening of his own to trap the torpedo. By now Blake's part of the plan is done, Unwin is inside the control room, the 250 hour mark is very near and Matthew is hard-pressed to get anywhere in trying to pull out the torpedo! Out of fear, the controllers of the plant respond to Father Unwin's urgent pleas to shut off the pumps. Matthew frees the torpedo and returns it to the saboteurs' sub. The explosion cripples the craft, the plant is saved and the police whisk away the villains — headed by the American ready to cancel his plant order (perhaps there was a rival U.S. firm designing plants of similar function!)

A daring prison break-out yields masterspy Grey for a gang of thugs. A helijet whisks Grey away and the in-prison-uniform homing device traces the journey for 50 miles then... nothing! Last monitored over the estate of Sir Humphrey Burton, the bishop puts Father Unwin and Matthew into operation. The Father makes a 'social call' on Burton and, in the Unwinese confusion, Matthew secretes himself away from detection. Here he begins to see all is not the same on the inside as it appears on the outside. Thanks to a chair which lowers down to a secret room, Matthew soon finds Grey, the control setup and, in an adjacent hangar, the missing helijet. Burton comes down, informs Grey of the second leg of his exodus and Matthew, after Burton leaves, tells Father Unwin the news. It is hoped that Matthew can accompany the men aboard the helijet and be able to gather in help along the way. The first leg of the journey takes the helijet to a farm and an awaiting car. With Matthew making the transfer to the car successfully and the Father discreetly behind, the car trip winds up in Kew Gardens. In the hot-house, Burton and Grey meet another contact who will lead Grey to an offshore submarine. Our heroes arrive and, whilst Grey and the contact are arrested, outside Father Unwin miniaturises Burton and a gun fight ensues: Matthew versus Burton in the plants! Burton gives up and joins his friends in the waiting police car. A scare for British defence security is averted.

Someone is getting away with murder! By taking photographs in broad daylight of a secret British aircraft, frustrating normal security procedures. A man, Masden, is being blackmailed to do so, supplying the pix to someone called De Groot who is suspected by the bishop. Soon Father Unwin and Matthew are blended into the mysterious picture but, unbeknown to them, De Groot is ordering Masden to fix it for a final run of photos of the XK-4 'plane and then to arrange for its destruction! Our heroes survey De Groot's house, follow Masden to it, use his vehicle number plate to find his address and pay him a visit. Whilst Unwin ties up Masden in verbal confusion, Matthew is able to search the grounds. He sights some bird coops and is chased up a tree by Kruger Masden's great dane! In the chaos, Father Unwin is able to do a little searching around himself. He finds the mini-camera equipment that has been doing the photographing. Meanwhile tests begin at De Groot's house with a radiocontrolled bomb release mechanism. At the same time Unwin and Matthew are starting to get closer to the real picture and the security man at the air base is getting more nervous. Soon the Father and Matthew work out the true picture-taking method and meet a remorseful Masden who outlines the final evil intent of De Groot. Together they race off to the airfield perimeter where De Groot is ready to attack. After a scuffle, Matthew wrests possession of the transmitter and is able, with Father Unwin's binocular device, to release the explosives away from the air field, save the jet, put De Groot away and, at the same time, teach Masden a lesson he knows only too well!

Father Unwin and Matthew are called in to safeguard a shipment of gold bullion aboard a train, after a security truck is ambushed in a previous attempt. As soon as they reach Buffler's Halt, a dusty deserted railway station, they realise that someone has diverted them there. It is the evil work of a man named Price who is using his British Rail position clandestinely to detour the train. A retired worker living in the area, Hobson, knows this and when the crooks open the train up at the Halt they discover Father Unwin, capturing him and the train crew. This brings Hobson and Father Unwin together to get to the bottom of this chain of events. Meanwhile, Matthew has been left with Price and Callow, the crooks. He creates confusion which diverts the hijackers' attention. This enables Hobson and Unwin to reverse roles and take the train away to safety before a convoy of trucks arrive to ferry away the bullion. All this while, Hobson, now in control of the train, encounters the different feel between a diesel train and the steam engines he was used to!

Poor Father Unwin is bedridden with an illness and, after a strong dose of medicine, he soon drifts off into a very heavy sleep. In his dream he imagines Matthew and himself flying medical supplies in Gabriel all the way to Central Africa! First stop is an elderly-looking petrol station for re-fuelling. They are taken care of by none other than Mrs. Appleby — just the way they like to be taken care of in reality. From here the two heroes fly to a jungle pick-up point to take possession of the medicine but, once there, they are met by hostile natives and held captive. Only when Unwin tries Unwinese do the natives understand and agree to release the pair! Once off on their way, with the supplies, there remains one last bump in the road, a blockade and a fight. The blockade, a missile airbase, throws everything the rather German-looking General can command: surface to air forest-based gunfire and a formation of pursuit fighters. All fail! The Father's flying causes the gunfire to miss and knock-out the base control buildings and the 'planes to collide with one another. Past this inconvenience, our two deliver the goods to Doctor Purple who is in charge of relief medical efforts... but he turns out to be someone (in this dream environment) to be none other than the Bishop! At this point Unwin shoots up in bed to discover that morning in the real world has indeed arrived.

Before a World Army Officer's demonstration, trials of the Experimental Vehicles Division's 'AquaTank' result in the near-destruction of the tank so Father Unwin is brought into the World Army Testing Base prior to the actual day of the demonstration. He gains entry to the Base by donning his old Padre uniform from early in his career and makes for the storage building housing the AquaTank overnight. Here Matthew will sneak aboard the tank to get an inside viewpoint of the vehicle's progress during the test next day. Father Unwin, already with permission to have free reign of the Base, is positioned on the perimeter should trouble occur. The next day sees Colonel Blair giving the assembled Officers the background on the AquaTank's high-tech remote control abilities and its progress is seen on a big screen monitor. The demonstration begins with professor Graham, the tank controller, issuing the computer program from a control truck beside the Control Bunker. Unbeknown to all, Captain Mitchell is the 'inside man' for whoever is out to halt the development of the tank. He sneaks away and locks everyone in the Bunker. Then, racing over to the Control Truck, he overpowers the professor and re-sets the target selection for the AquaTank to the Bunker itself. Matthew needs little time to see the change and relay such in formation to Father Unwin who races to the truck in Gabriel amidst the pre-arranged minefields and gunfire from strafing aeroplanes. Once at the Control Truck he is able to convince the recovering professor of the need to tell him how to wrest control of the AquaTank back to manual control. Once the information has been gleaned, Matthew is able to struggle with the heavy controls and turn the tank away from the Bunker just in time. The shell fires and bull's-eyes one of the target buildings which Captain Mitchell thought would be a good hiding place!

Two security officers on a golf course pass satellite orbit control-changing frequency information, unaware that it has been electronically recorded on their golfball! Once in the tee's hole it travels to an underground lair and saboteurs who use the information to jam the transmitted signal up to the G-9 satellite still on a decaying orbit. Should it come to a fiery end it would be a severe blow to British Intelligence. Call in Father Unwin and Matthew. Spare boosting components to reach the G-9 are to be sent by transport truck and this information is recorded by the golf ball and used by the sabotaging duo to blow up the truck en route to the Early Warning Tracking Station. Returning from the links, General Brompton finds the Bishop himself in his ransacked office looking for a bugging device that may have been used. The next day Father Unwin meets up with the General at the Club House and a friendly game of golf is agreed. On the course Father Unwin is passing dummy information on to Brompton and the 'ball' picks it up... leading the opposition to a caravan which they blow up. Matthew sneaks into the boot of their car for a free ride back to the hideout. Once in the lair, Matthew sees the system in action and when alone relays this ground-breaking information to Father Unwin. Unwin is on to the Bishop for a special golf ball of his own to help stop the next jamming manoeuvre. Later the same morning the Father and the General play through to the infamous 15th Hole. Matthew is able to open the bottom of the false hole in order to receive the special ball. It is now up to Father Unwin to play one final shot. The ball drops in the hole and down to the lair, exploding in a fuming cloud of knock-out gas and preventing the jamming. The G-9 sails back into a fresh orbit and the authorities swoop down on the saboteurs, mission successful!

Father Unwin and Matthew are called in after an Experimental Vehicles Division complex is destroyed. Of interest to the Bishop is that after a mission to destroy an explosives truck an injured priest is found close to the sabotage... and that a priest is seen close to the previous operations. Father Unwin pays a visit to the injured priest, gaining a lot of information about the next mission and the sabotage set-up whereby gang members use Brother or Archdeacon prefixes to their names! Matthew is brought to the hospital room and hides himself in the hand luggage of a visiting gang member, guaranteed of a return trip to the seminary hideout. Unwin must now replace a Brother Jesse on his way to the seminary to help in Operation Fish... the next major offensive. The switch is made as the real Jesse is detoured into the arms of waiting police. Showing up at the seminary with information Matthew gathered from breaking into the safe the night before, Father Unwin is met by the gang as Brother Jesse. Here he must fake his knowledge of explosives for the mission but, as he gains their trust, the injured priest arrives at the seminary from hospital only to inform his Archdeacon, the ringleader, of the phoney nature of 'Brother Jesse'. Secretly, the gang plays along with Unwin until the night of the mission — to destroy the submarine Invincible in its sheltered inlet base. When the truth is known Father Unwin and his case are put in the explosive-packed speedboat. Target: the submarine! Somehow Matthew is able to pre-detonate the explosives while the Father treads water nearby, having lost his perch on the side of the boat. The submarine is saved but Father Unwin and a sneezing Matthew return to round-up the Archdeacon at the seminary, putting him out of action with a phial of nitroglycerin, all whilst the rest of the gang are being rounded-up!

Father Unwin and Matthew are called in to investigate the presence of foreign agent Sakov at Doctor Klam's Health Centre. The Centre is next door to a race track where a new fuel is being tested, one that, it is hoped, will be able to be sold into international markets. Arriving at the Centre as a patient whilst Matthew in miniaturised state stakes out the race track, Father Unwin is strapped into the Klam Therapeutic Recliner, designed for back therapy. Klam, a student of stern treatment, puts Unwin into the routine and removes his vital hearing aid. At the same time Sakov comes along and proclaims his true intent, that of sabotaging the fuel testing. In front of a Government Minister a car with the old fuel will use this up and then switch to tbe new fuel, Matthew being inside the car. Father Unwin, unable to contact Matthew and vice versa, pleads with Doctor Klam for his release but only manages it after pleading with a sympathetic nurse. Using the Minimiser, he reduces himself and Gabriel to escape through a small hatch in a locked door. Previously, Sakov has shot out the tyres of another race car, hampering the tests but the latest will be saved as Father Unwin arrives at the test pitstop! Grabbing a walkie-talkie from one of the trackmen, he passes the urgent information to Matthew who is able to stop the car manually before the tampered-with new fuel is scheduled to come into use.

Father Unwin is allowed by the Bishop to check on an old friend, Professor Soames, whose common interest with the Father is gardening. It seems that Soames has changed his favourite growths to some new variety of plant. He also lives near an airfield testing a new aircraft, hence the security angle and the interest of the Bishop. Matthew, left behind after the visit, soon finds out that Soames and his daughter are being held captive by a gang interested in using the Professor's newly designed sonic rifle which uses sound as a weapon. Their target is the new aircraft. They can use the gun against it and the blame can be placed on a design fault, not outright sabotage. A test flight is scheduled for that very evening so the gang spring into action... and so do our heroes! Unwin allows himself to be brought into the captives' circle and when two of the three captors leave to set up the rifle, Unwin and Soames overpower the other gang members. The Father and the professor are unable to track the other two who are free to set the gun up almost anywhere near the airfield so the only answer is to blanket the area with more sound than would be generated by the actual weapon. Hence a request for a hypersonic aircraft to overfly at ultra-mach speed. A sonic back-lash is created and the gun blows up as a result, leaving the other gang members merely unconscious and ready for arrest!

An Arabian King is to fly to London to sign an oil deal then jet over to New York soon afterwards. Those opposing the deal have already tried four times to assassinate the King, trying to end the deal which would bring the King's country an economic and social boost. In London the King is accompanied by Father Unwin and minimised Matthew. In the leader's hotel room, Matthew is ready to alert Father Unwin to any more attempts. Such a bid is made one evening, yet Matthew's miniature presence scares away a knifewielding thug, part of the gang from the King's country. Father Unwin soon attends to tell of the attempt and that it has been foiled. The next attempt will happen over the Atlantic as the gang of would-be killers have a microbomb planted in a teddy bear, to be presented as a gift prior to take off. Seeing the presentation on the TV news, the bomb supplier rushes off to inform the police but is shot down on the way. The 'plane takes off and the dying informant's story is passed to the Father. Matthew finds the bomb in a cargo hold and requires the hatch open to throw the bear out but Unwin goes to the cockpit only to discover the pilots have been gassed out. Matthew is able to ditch the bear-bomb but there remains the problem of landing the 'plane in Iceland. As a landing at sea is out of the question, due to choppy weather, Father Unwin must and finally does, land at the airport. Talked down by the local air traffic controllers, the King sleeps through all the ongoing drama!

Matthew and Father Unwin are put on the trail of a released counterfeiter. If successful, they will be lead to his cronies and the whole gang could be arrested in one fell swoop. Unwin and Gabriel both join Matthew and are minimised. Mullins soon finds himself renewing his association with Lord Hazlewell, his sister Lady Martha and a 'normal' criminal named Spiker. They make an oddball assortment and soon start to try and take various forms of advantage of each other. The Lord and Spiker try to get Mullins to tell them where a valuable missing mint plate, used in counterfeiting, is but they are not alone as our heroes and Lady Martha, are listening. Once she knows she locks the others in a dungeon and makes off for the plate with Unwin and Matthew in minimised pursuit, even after she changes from a car to a tough's motorcycle! Lord Hazlewell and Mullins soon escape and trick Spiker into leaving them clear to join the chase in a bi-plane. Spiker joins the race in an ambulance, knowing the other two can't fly the 'plane for their life's worth! As all the participants arrive at the barn where the plates are hidden, after many amusing near-disasters en route, they scramble for the plates they seek. However Father Unwin and Matthew use their minimised state to claim the plates for themselves, whilst the other four bicker amongst themselves.