Moominvalley is hit by a terrible storm, and an old sailing ship is wrecked offshore, finally sinking. But, it is no usual ship - she is seen flying a black flag with a skull and crossbones. The pirates have made it to the shore unhurt and trouble starts when the captain, a terrible villain, grabs Snorkmaiden and holds her hostage, demanding a new getaway ship from the people in Moominvalley. The Inspector is frantic and helpless and it is left to Moomin to bring about her rescue.