The player commands USS Idaho, a fictional Ohio-class ballistic missile submarine. On a routine nuclear deterrence patrol, an encoded message is received from U.S. COMSUBLANT. The message reports that a stolen Libyan diesel sub has exited the Mediterranean Sea, possibly heading into Idaho's area of operations. Idaho must evade the potentially hostile submarine, a problem that is quickly complicated by the fact that the enemy submarine is equipped with sound-generating equipment that allows her to mimic other classes of submarine, including those of American design. The rogue Kilo-class sub uses this tactic to launch a torpedo attack on the Idaho by pretending to be an allied Los Angeles-class sub, USS Biloxi.