John Hart (The Sea Devils)

Captain John Hart of the Royal Navy worked at the secret naval research centre, HMS Seaspite.

Third Officer Jane Blythe worked as his secretary.

Biography
When the Third Doctor arrived at the base, Hart telephoned security when he saw him on the beach. The Doctor was brought to Hart and explained he was from UNIT. After Jo Grant was able to clarify this, Hart 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. Hart brought the Doctor and Jo aboard the diving vessel the Reclaim, where the Doctor was lowered into a diving bell but was captured 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, 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)