Captain John Hart of the Royal Navy worked at the secret naval research centre, HMS Seaspite. Jane Blythe worked as his secretary.

The Third Doctor arrived at the base and Hart telephoned security when he saw him on the beach. He was brought to Hart where he explained he was from UNIT. After Jo Grant was able to clarify this he let them go.

Hart refused to have the Doctor taken to a sea fort station, describing it as "provid[ing] navel vessels for [his] pleasure jaunts". After the Doctor and Jo were reported missing he telephoned for an air sea rescue helicopter. He brought the Doctor and Jo aboard the diving vessel the Reclaim, where the Doctor was lowered into a diving bell and taken to the Sea Devil base.

Parliamentary Private Secretary Walker ordered Captain Hart to carry out the attack on the base, with the Doctor still inside. After the Doctor escaped, he hoped to return to the base to try one more negotiation with the Sea Devils, when the Sea Devils captured HMS Seaspite. When the Doctor incapacitated the Sea Devils with a sonar device, this created a distraction for Jo and Hart to escape HMS Seaspite on a hovercraft and head out to sea to reclaim the naval base with naval reinforcements.

Walker ordered a nuclear strike on the Sea Devil base, which the Doctor had returned to. Hart ordered a sweep of the base by a high-speed hovercraft to see if the Doctor made it out. The hovercraft picked up the Doctor and the Sea Devils' accomplice the Master, and the nuclear strike was called off when the Doctor created a reverse feedback in the base's power system that destroyed it. (TV: The Sea Devils)

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