Three men begin their six-week shift tending to the remote Flannan Isles Lighthouse. Donald (Connor Swindells), the youngest, is inexperienced and learning the trade from James (Gerard Butler) and Thomas (Peter Mullan). James has a family waiting for him on the mainland and Thomas is still mourning the loss of his own wife and children. After a storm, the men discover a boat, a body, and a wooden chest washed ashore. Donald descends the cliffs to check on the man, who appears lifeless. As they are hauling up the chest, the man awakens and attacks Donald. In self-defense, Donald bashes the man's head with a rock and kills him.