The hypersonic airliner Fireflash 3 takes off from London International Airport as per usual, but later sends out a mayday, reporting losing height rapidly and reporting their position as TS757/AR436. London Airport sends out the Air Sea Rescue squad who fail to find any trace of the craft at its last reported position. The crash makes headlines, and the Air Ministry has the entire Fireflash fleet grounded until further notice. Meanwhile in Thunderbird 5, Alan Tracy realizes that Fireflash 3's last reported position was incorrect, more than 50 miles out, explaining the inability to locate the downed craft. Patterson, a technician of the Fireflash division, tests the Fireflash craft, but everything checks out, so a test flight is launched the next day. All goes well, until the craft approaches the last position reported by Fireflash 3. The EPU (Elevator Power Unit) reads zero despite still being 150,000 ft. in the sky, and all the instruments start going haywire. Fireflash then sends out a distress call to London Airport, reporting mechanical problems, and eventually losing all contact once it reaches the exact position where Fireflash 3 disappeared. The pilots are then unable to control the craft as it dives towards the Atlantic, smashing into the water, but still in one piece. The pilots desperately try to escape the flight deck, but the escape door jams, leaving them trapped inside as Fireflash sinks beneath the surface of the water. An unidentified man (who parachuted from the undercarriage of the Fireflash prior to it sinking,) is then picked up by an EJ2 Jet; it was sabotage.
Alan calculates that the sunken Fireflash's reported position was again off by approx. 180 miles northwest of position TS749/AP428. Thunderbird 2 is then dispatched to the position given by Thunderbird 5, and Brains calculates that the aircraft's mechanical failure could've prevented the crew from escaping, meaning the crew is trapped in the cockpit at the bottom of the sea. Meanwhile, on the sea bed, the crew desperately try to call for help, but to no avail. Thunderbird 4 is launched to explore the sea bed for Fireflash, and Gordon Tracy makes contact with the crew. He then uses Thunderbird 4 to cut off the heavy engines holding Fireflash down. Fireflash then floats to the surface, when the control panel suddenly bursts into flames, meaning Gordon must work fast. The crew is freed, and Fireflash erupts in a massive explosion. International Rescue is called to assist in the next test flight for another Fireflash, with Scott Tracy at the helm, and Gordon and Virgil in Thunderbird 2 to monitor Fireflash and intervene if necessary. As they approach the "crash zone" again, Scott radios his coordinates to Thunderbird 5 as position LS749/AP428. Alan tells Scott that the position is wrong yet again, 20 miles northwest, and Gordon boards Fireflash to inspect any damage. He finds a large wired coupling that has been crudely cut in half, and he is then fired upon by a saboteur working for Benton Aircraft Espionage. As this happens, Fireflash starts its sharp descent towards the ocean. The saboteur then jumps out through an opened undercarriage hatch, even while being shot at by Gordon. As Fireflash nears the water, Scott tells Gordon to fix the broken coupling but Gordon tells him it can't be done in time. Realising his only option is to recreate the circuit, Gordon grabs the wires and forces them together, giving control back to Fireflash. Fireflash returns safely, and the threat of sabotage wasn't heard from again. The Fireflash fleet goes back into operation, and the Tracy boys head for home.