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...

Amy Pond has been kidnapped and the Doctor is raising an army to rescue her as the drama continues. But as he and Rory race across galaxies, calling in long-held debts and solemnly given promises, his enemies are laying a carefully concealed trap.

In her cell in Stormcage, River Song sadly acknowledges that the time has come at last – today will mark the Battle of Demons Run and the Doctor’s darkest hour. Both sides will make their sacrifices and River Song must finally reveal her most closely guarded secret to the Doctor.