It’s 1944 and the S240 submarine – commanded by Captain Emmett Carey (Scott Brady) – has been searching the waters of the South Pacific for a sign of the missing S342 submarine (aka the “Gray Fin”).
The sub suddenly receives orders to ferry a six-man Underwater Demolition Team (UDT) headed by Lt. Morgan Hayes (Tab Hunter) to the Marshall Islands to destroy the remains of the Gray Fin where it has sunk in the Bikini Atoll.
This entails the S240 sailing a hundred miles through enemy waters to an occupied area where guerillas will then direct the UDT team to the wreck.
The sub’s crew – among them executive officer Lt. Bill Fortney (Michael Dante), Lt. Cale (David Landfield), and trouble-making Bosun’s Mate Fennelly (Jim Backus) – are resentful that they have to take themselves out of the action.
Hayes himself does not know why they have been sent to blow up the remains of a sunken sub, but Carey informs him that the S342 was carrying a prototype microwave radar system and they want to prevent the Japanese from salvaging it.
Tensions rise between the sub crew and the UDT team – which includes short-fused Givens (Gary Crosby, son of Bing), rookie Malzone (Frankie Avalon), translator Davayo (Aki Aleong) and Sherman (Jody McCrea) – but they get it all out of their system with a fistfight.
When they reach the Bikini Atoll and find the lagoon filled with Japanese ships that have just started the salvaging operation on the sunken sub, Hayes and company realise they will have to team up with the crew of the S240 for a dangerous Plan B.
Although the movie is in black and white, it segues to rather restrained colour for a painful extended dream sequence in which Avalon sings The Girl Back Home – about being torn between his girl back home Roxanne (Nancy Dusina) and a sequined siren (Judy Lewis). The film segues back to colour again for a beachside bikini romp under the end credits.
Lt. Morgan Hayes
Capt. Emmett Carey
Chief Bosun’s Mate Fennelly
Lt. Bill Fortney