[1][2][3] It is the third and last of the Secret Agent 077 film series starring Ken Clark as 077 and Daniela Bianchi as fashion expert and professional killer Lady Arabella Chaplin.

CIA Secret Agent 077 Dick Malloy (Jack Clifton in the German release) is sent to Madrid when someone wishes to sell a dog tag apparently recovered from the sunken American nuclear submarine USS Thresher.

Once the item is identified as a genuine article, the man selling the information is killed before he can reveal how he acquired the dog tag from the ocean floor.

Malloy and his boss Heston interview the Howard Hughes type marine salvage multi-millionaire Zoltan in order to determine whether the wreck of the submarine could be accessed.

Using an advanced bathysphere, Malloy travels to the site of the remains of the Thresher and discovers that the 16 Polaris missiles with nuclear warheads carried by the submarine are missing.