The British nuclear submarine HMS Proteus carries the most sophisticated anti-detection equipment in the world, codenamed "Leopard". Her first mission is to chart the new Soviet underwater defence system which poses the most deadly threat to NATO's northern flank.
But Proteus picks up a distress signal from a Soviet submarine lying crippled on the sea floor. And as she sets out on her rescue mission, the Russians prepare to spring a devastating deathtrap twenty fathoms beneath the ocean. The plan: kidnap the Sea Leopard!