On his way home from work, Thomas Prescott stops to pick up a hitchhiker who claims to be going to visit his doctor. However, when he arrives at the destination, the mysterious man attaches a strange bracelet to Prescott's wrist, pulls out a gun and reveals that he knows an awful lot about the driver. Prescott is told to go to his office and find a certain file, which contains the key that unlocks the bracelet, then leave the bracelet behind. The bracelet will explode at eight o'clock pm, so he is encouraged to hurry as it is now twenty minutes to the hour.
Trapped In A Lift[]
Prescott.
Prescott follows the directions, catching the attention of a police car as he speeds away. Although he finds the key and removes the bracelet in time, it explodes while he is still taking the elevator to the ground floor. With the cables severed, the lift plummets into the lowest basement. Prescott is trapped inside and the lift shaft gradually begins to fill with water as the fire crews tackle the blaze caused by the explosion, the automatic fire-fighting equipment in the building having been disabled. Lacking the equipment and manpower to save him, the firemen call in International Rescue.
Dicetylene Cage To The Rescue[]
Scott in Thunderbird 1 and Virgil and Alan in Thunderbird 2 hurry to the danger zone, where they set up the Dicetylene Cage in the lift shaft. The cage lowers Virgil and Alan
down to Prescott's elevator car, spraying dicetylene powder to put out the fire that has started to spread down the shaft. Once they reach Prescott's car, the Dicetylene Cage grabs hold of it and lifts it back up the shaft. Unfortunately, the dicetylene runs out on the way back up, leaving Prescott exposed to the open flames. He makes it safely back to ground level, however, where Scott helps him to safety.
Undercover Operation[]
The next day, the Tracy family are shocked to read in the newspaper that Prescott has been declared killed in last night's fire, even though the rescue was a success. Jeff concludes that it must be part of an undercover operation, and he is right: back in England a secret service agent named Southern (call sign: Tiger Four) is receiving the assignment to infiltrate the gang responsible for the bombing.
He and the villains are armed with the explosive bracelets and sent to their next target—a high-security plutonium storage facility—with the intention of causing a nuclear disaster. The gang penetrates the many security doors, disarms the robot guards and finds the key to the main vault, which also fits their bracelets. At that moment, Southern reveals himself and holds them at gunpoint, but a last security robot sneaks up and grabs him from behind. The gang members seize their chance and leave Southern with the explosives, which will explode at thirty minutes after noon, closing and disabling the security doors behind them.
No Escape[]
Southern calls his superiors to have them capture the escapees, resigning himself to his fate, but his chief calls International Rescue to save him. Soon Scott and Virgil arrive and Thunderbird 2 unloads the Laser Cutter Vehicle to cut through the thick doors. They reach Southern with only minutes to spare. Virgil works to free Southern while Scott leaves the scene with the bracelets. He flies out to sea in Thunderbird 1 and drops them into the water a split second before they explode harmlessly.
Helijet Rendezvous[]
With Southern freed, International Rescue turn their attention to the escaping villains, who are rendezvousing with their boss at a helijet in a remote field. Just as they are about to take off, FAB 1 bursts through the hedge carrying Lady Penelope and Parker, who shoot down the helijet and all its passengers. Penelope meets up with Scott, Virgil and Southern and offers to take Southern back to her manor where he can recover from the injuries inflicted by the robot. There he laments that his career is over now that his cover has been blown and tries to stifle Penelope's apparent fascination with life as a secret agent, oblivious to her role within International Rescue.