Lady Penelope opens a pass in Australia for her friend, Sir Harry, who is the head of the project. She takes the opportunity to go to her farm, and invites Jeff to join her. He doesn't want to go, in case there is an emergency, but she and the boys insist that he takes a break. He leaves Scott in charge, and John mentions before Jeff leaves that the World Navy is on exercise in the Atlantic.

The Seascape drilling rig in the Atlantic has been assured that they are at no risk from the military exercises, but a stray nuclear missile detonates on the sea floor close by, boiling the water in the area.

Meanwhile, Jeff has arrived on Lady Penelope's sheep farm, and has strict instructions that he is only allowed to make one call home during his stay.

An explosion on the seabed hear the Seascape rig ignites a fire jet from the ruptured gas field below. Scott questions whether or not to get involved. He makes the decision to deploy International Rescue, whilst Jeff, on Penelope's farm, has deemed this not to be a job for the organisation.

Alan lands Thunderbird 1 on the Seascape and sets up the Mobile Control. Gordon takes a sealing device down to the breach and the fire jet is extinguished. However, the seismograph is picking up violent tremors in the area.

Jeff, having heard about Scott's decision, calls home and tells Scott that he is coming home straight away, but Penelope persuades him to stay.

Meanwhile, another fire jet erupts. Scott decides against action, following his father's objections, but other fires erupt and the Seascape is damaged. Two men, O'Shea and Hooper go down in a diving sphere to assess the damage, but another explosion causes the cables holding the sphere to snap, dropping them to the bottom of the ocean. The Thunderbirds are deployed again.

Lady Penelope and Jeff hear a newsflash about this. Jeff decides that the boys need his help, and leaves with Penelope in tow.

Navy helijets lift the crew off the rig, but are unable to rescue the two men trapped in the diving sphere. Unfortunately, the rig continues to collapse and Alan is forced to lift off before the mission is complete, before Thunderbird 1 slips into the sea.

Gordon manages to free the sphere and tow it away from the area before the rig explodes. Virgil takes the sphere to safety, momentarily abandoning Gordon.

Jeff arrives home, but Scott refuses to give him permission to land, as the Thunderbirds are due back!

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