"The Mysterons (voiced by Donald Gray) announce that they intend to destroy London. Aided by Captain Black, they use their powers to seize control of a transporter truck carrying an atomic device through the city. The transporter and its driver, Macey, are sealed inside Park View, an underground car park. Macey, who was knocked out during the hi-jacking, wakes up not knowing where he is. Turning on his radio to hear Big Ben strike midnight, he is surprised to hear 13 chimes instead of 12. The Mysterons start the device's 12-hour detonation countdown, knock out Macey for a second time and then dump him in a side street.
Following the disappearance of the device, Spectrum is put on red alert. Macey is found by Captain Scarlet (voiced by Francis Matthews) and taken to Cloudbase to describe his ordeal to Colonel White (voiced by Donald Gray). More than 2,000 London car parks match Macey's description of Park View. Remembering the driver's claim that Big Ben struck 13, Captain Blue (voiced by Ed Bishop) narrows the field down to two candidates – one of them Park View – by calculating that Macey's car park cannot be more than a mile from Big Ben.
Less than an hour before detonation, Scarlet and Blue fly to London, speed to Park View in a Spectrum Pursuit Vehicle and locate the transporter. With too little time left to defuse the device, White orders the officers to take the transporter to a construction site – its intended destination – and leave the device to detonate in a specially prepared excavation below ground. Scarlet and Blue reach the site with just minutes to spare. With Scarlet driving the transporter, Blue lowers it into the excavation in a lift. Scarlet abandons the transporter and starts back up, but moments later the device explodes, destroying the lift shaft. Scarlet is fatally injured but recovers thanks to his retro-metabolic powers.
Some time later, while dining at a restaurant with Scarlet, Destiny Angel (voiced by Liz Morgan) and Melody Angel, Blue explains how he was able to work out the car park's approximate location and why Macey heard Big Ben seemingly strike 13. Macey was hearing two sets of chimes: one over the air, the other on his radio. However, the airborne chimes were delayed by the speed of sound, so the "thirteenth" chime was actually a repeat of the twelfth that had already sounded on the radio. Following this explanation, Scarlet decides to make 13 his "lucky number"."