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