'Plant of Doom' by Alan Fennell

Titan is furious that his slave, Marina, helped Troy and Phones to escape from Aquatraz. To gain his revenge, the angry ruler instructs his surface agent, X20, to deliver a beautiful plant to Marina's father as a gift. Later, a homesick Marina visits her father, and then takes the flower back to Marineville. However, Atlanta unwittingly breathes in the plant’s toxic fumes, and she immediately collapses; her father, Commander Shore, suspects that that Marina is a spy - and so, in order to discover if she is guilty or innocent, he sends the undersea girl into the room where the deadly plant lies…

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), David Graham (Aquaphibian)

Directed by David Elliott
Produced by Gerry Anderson

TX (UK): 23rd May 1965

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