The Moomin's first night in the abandoned lighthouse is haunted by ghosts from the mysterious ghost ship, who finally take off. One of their group is left behind, and the Moomins have to build a raft for it to sail away on. Moominpappa is in vain trying to light the beacon in the lighthouse and they all try to unravel the mystery with the absent keeper. It is finally solved with the help of the small boy, Toft, from the fishing boat, and when the Moomins sail back to Moominvalley they see the lighthouse lit for the first time.