It is 1962, and Marta Khoury, a trailblazing marine archaeologist, has been called to Cairnroch, a small island off the east coast of Scotland. A Victorian shipwreck holds the remains of a celebrated explorer and the treasures of his final expedition. But on her first dive down to the ship, Marta becomes convinced she has seen a dark figure lurking amid the wreckage. When the Cuban Missile Crisis and the deep chill of a record-breaking winter keeps Marta stranded on Cairnroch, she forms a relationship with Elsie, a local woman working in the island’s only hotel. When the ship's artifacts inexplicably disappear, Marta and Elsie have to brave the freezing conditions to search for the missing objects before anyone else catches on.