Billy Bosun is a retired mariner living in a quiet coastal village, whose peaceful but increasingly boring life is interrupted by a young woman named Miss Malone. She identifies herself as Molly and implores Billy to help rescue her grandfather, Harry Prince, from a mysterious and inhospitable island. Billy initially resists the request until he is reminded of an old debt he owes to Harry, his former shipmate and friend. Billy reluctantly agrees to navigate the dangerous waters in his small boat. After a perilous journey they arrive on the island.