While flying from London International Airport to San Francisco,
Fireflash 3 is crippled by a technical fault and disappears over the Atlantic Ocean. Rescue teams find no trace of the airliner, which was carrying 600 passengers and crew. On
Thunderbird 5,
Alan Tracy determines that the
Fireflashs last radioed position was out by . By order of the International Air Minister, all
Fireflashes are
grounded pending a full safety review. No design flaws are uncovered, so Commander Norman orders a transatlantic
test flight to identify the fault. During the flight, the
elevator power unit, locator and radio fail, and the
Fireflash loses altitude and
belly-flops into the ocean. The emergency exit jams, trapping the crew, and the aircraft sinks. An unidentified man, who parachuted from the undercarriage, is picked up in a helijet. Alan alerts
Tracy Island, reporting that the
Fireflash went down from its last radioed position.
Jeff orders an undersea search-and-rescue mission, dispatching
Scott in
Thunderbird 1 followed by
Virgil with
Gordon and
Brains in
Thunderbird 2 carrying Pod 4. Brains believes that the
Fireflash sank intact and that the crew may still be alive, trapped on the seabed and running out of air. Landing at a farm in Ireland, Scott performs an electromagnetic sweep and plots the
Fireflashs position. Gordon launches in
Thunderbird 4 and locates the airliner. Using
Thunderbird 4s laser beam, he cuts off the
Fireflashs heavy
engine nacelles to enable it to float to the surface. As the cockpit instruments short-circuit and burst into flames, Gordon mounts the fuselage and cuts a hole in the window using a handheld laser, freeing the crew. Virgil airlifts the crew to safety, and Gordon escapes in
Thunderbird 4, moments before the
Fireflash is destroyed in a series of explosions. Air chiefs agree that the source of the
Fireflashes trouble lies in their wing hydraulics. Realising that another test flight is needed, Jeff contacts Commander Norman and recommends that
Thunderbird 2 accompany the aircraft so that if the fault reoccurs, International Rescue can respond. The
Fireflash takes off, piloted by Captain Hanson and Scott and flanked by
Thunderbird 2. Within minutes, the elevator power unit and other systems fail and the airliner begins to plunge towards the ocean. Hanson and Scott open a service hatch and Virgil steers
Thunderbird 2 underneath the
Fireflash, allowing Gordon to winch himself aboard. He finds that the elevator power unit cables have been cut, at which point the saboteur seen parachuting into the ocean comes out of hiding armed with a handgun. After a brief firefight, the saboteur tries to escape but is gunned down and falls out of the hatch. Moments before impact, Gordon presses the severed cables together, restoring power and enabling Scott and Hanson to pull the
Fireflash out of its nosedive. Within days, all
Fireflashes are back in the air and the authorities have shut down the criminal organisation behind the sabotages. On Tracy Island,
Grandma complains of a blown fuse, leading to Gordon's skills in fixing broken electronics being called on once again. ==Regular voice cast==