CIA Secret Agent 077 Dick Malloy is sent to Madrid when someone wishes to sell a dog tag apparently recovered from the sunken American nuclear submarine USS Thresher (SSN-593). 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#Glomar Explorer type marine salvage multi-millionaire Zoltan in order to determine whether the wreck of the submarine could be accessed. Zoltan denies it is possible but Heston and Malloy do not believe Zoltan is telling the truth.