While visiting Lady Penelope in England, Brains watches TV coverage of the forthcoming launch of the Martian Space Probe (MSP). Because Mars is currently in a tricky position relative to Earth, the countdown is set to occur from England rather than Cape Kennedy. Two technicians, Bill and Frank, are aboard the probe rocket, performing pre-flight checks as it is hauled to the launch site on the back of a transporter truck. Disaster strikes when the Allington River Suspension Bridge, weakened by a recent storm, collapses under the weight of the shipment. The rocket sinks to the riverbed, buried beneath wreckage, and the two technicians inside the nose cone learn the grim truth: the impact has triggered the probe’s automatic countdown, meaning that in twelve hours the rocket will spring free from its compromised position and surely be destroyed.
Meanwhile, on Tracy Island, Grandma misplaces her edible transmitter—an important homing device— prompting Jeff to suspect that one of his sons may have eaten it. After some misdirection and humorous tension, the culprit is revealed to be Jeff himself, who must awkwardly drink a transmitter-dissolving medicine prepared by Tin-Tin.
Brains and Penelope race to Allington in FAB 1, with Parker at the wheel. In a bid to buy time, Penelope and Parker fire the car’s machine gun at abandoned warehouses to distract the emergency services, allowing Brains to slip into the bridge control room. There, the personnel plan to hoist the rocket from the river using a floating crane, but the Controller refuses Brains’ suggestion to summon International Rescue, insisting he stay out of the way. Undeterred, Brains secretly reaches out to the Tracys using his wristwatch videophone, which astonishes the crew—who mistake it for Brains merely talking to a watch.
Jeff quickly mobilizes reinforcements: Scott, Virgil, and Gordon are dispatched in Thunderbirds 1 and 2, with Gordon aboard the craft carrying Thunderbird 4. At the bridge, the plan to recover the nose cone encounters trouble when the crane is unable to manage the rocket’s weight, causing it to capsize and sink once more.
The Tracy party arrives to take action. Thunderbird 4 is deployed, and Gordon uses the plasma torch to clear away wreckage while Virgil lifts debris with Thunderbird 2’s electromagnetic grab lines. Time is running out, and the nose cone remains stuck beneath the river’s surface. Brains proposes a bold, faster solution: torpedo the wreckage to scatter it, then ram the nose cone free. The risky maneuver succeeds, the nose cone rises, and Thunderbird 2 hauls it away. With the nose cone secured, the rocket finally lifts off, punches through the river’s surface, and detonates in the sky.
In the aftermath, Brains celebrates with a quirky dance in the control room, which raises questions about his mental state from the about-to-be-confirmed psychiatrist, Dr. R.G. Korda. Penelope arrives to take Brains home, while Korda remains perplexed by Brains’ unconventional habit of “talking to watches.” As the scene closes, Penelope’s compact—really a concealed transceiver—continues to call the Tracys, leaving Dr. Korda unsettled about what he has witnessed.
