The TARDIS is marooned onboard a 17th-century pirate ship whose crew is being attacked by a mysterious and beautiful sea creature. Becalmed and beset by cabin fever, the pirates have numerous superstitious explanations for the Siren's appearance. The Doctor has other ideas but, as his theories are disproved and every plan of escape is thwarted, he must work to win the trust of the implacable Captain Avery and uncover the truth behind the pirates' supernatural fears – and he must work quickly, for some of his friends have already fallen under the Siren's spell...