'Hostages of the Deep' by Alan Fennell

When an undersea alien named Gadus kidnaps World Navy Admiral Carson and his wife from their remote island home, Troy, Phones and Marina head off on a rescue mission in Stingray. With the help of a coded signal from the Admiral, the crew escape being killed by a booby trap, but when they attempt to follow Gadus’ submarine, the villain dives into a deep trench too narrow for Stingray to enter. Marina sets off in pursuit on her own, only to be captured by Gadus; Troy and Phones set off to rescue their friend - but will they be in time to prevent the young girl from being impaled by a swordfish’s razor-sharp sword..?

Don Mason (Captain Troy Tempest), Robert Easton (Lieutenant George Lee ‘Phones’ Sheridan / Agent X20), Lois Maxwell (Lieutenant Atlanta Shore), Ray Barrett (Commander Sam Shore / Lieutenant Fisher / Titan / Gadus), David Graham (Admiral Carson / Maren), Sylvia Anderson (Millie)

Directed by Desmond Saunders
Produced by Gerry Anderson

TX (UK): 13th June 1965

Notes:
*Featuring Captain Troy Tempest, Lieutenant George Lee 'Phones' Sheridan, Commander Sam Shore, Lieutenant Atlanta Shore, Lieutenant Fisher and Marina