In December 1986, the Swiftsure-class submarine, HMS Saracen sailed from Devonport for a routine patrol within the Arctic Circle. A series of catastrophic incidents combine to leave her crippled on the bottom of the Barents Sea, dangerously close to the Russian coast. The crew struggle for days to repair the boat before their life support systems fail. Eventually returning to the surface, the world is a very different place. Inexplicably, their nuclear submarine has been transported back in time to witness a historic sea battle. A series of questions now face the captain; how have their rules of engagement changed? Can they change history? More importantly, how are they going to get home again?