Marlowe Chan-Reeves – Ying Ki
Crystal Yu – Madam Ching
Craige Els – Chief Sea Devil
Arthur Lee – Ji-Hun
David K. S. Tse – Ying Wai
Jon Davey – Sea Devil
Simon Carew – Sea Devil
Chester Durrant – Sea Devil
Richard Price – Sea Devil
Mickey Lewis – Sea Devil
Andrew Cross – Sea Devil
Nadia Albina – Diane (uncredited)
Directed by Haolu Wang
Written by Ella Road and Chris Chibnall
Script editors: Caroline Buckley, Rebecca Roughan
Produced by Nikki Wilson
Executive producer Chris Chibnall
Incidental music composer: Segun Akinola
Running time 48 minutes
First broadcast 17 April 2022
In 1807 China, the pirate Madame Ching raids a village and, using a glowing gem, unwittingly releases the Sea Devil Marsissus from being trapped as a stone statue. Marissus then kills Ying Wai, the young Ying Ki's father, whose task had been to keep Marissus contained. The Doctor, Yaz, and Dan arrive to confront Marsissus and investigate the village. Ying Ki and Dan take the gem, split off, and swim out to Ching’s ship to take revenge. Ching reveals she is after the treasure of the legendary sailor Ji-Hun (who disappeared searching for the lost Flor de la Mar treasure) because she needs it to pay the ransom for her kidnapped crew, including her two young sons. The Sea Devils unleash a leviathan, the Huasen, on Ching’s ship.
The Doctor and Yaz travel back 274 years and witness a Sea Devil betraying Ji-Hun aboard the latter’s ship. Returning to 1807, they try to find the shipwreck to no avail. The Huasen takes the TARDIS, with the Doctor and Yaz inside, to the Sea Devils’ undersea lair. The Sea Devils seek the gem, the Keystone, to execute their plan. Pretending they have it, the Doctor and Yaz use it as leverage to board Ji-Hun's ship, which has been retrofitted with alien technology. They are shown Ji-Hun, who has been kept alive in stasis. He reveals he tricked the Sea Devils in their deal. Marsissus calls the Doctor’s bluff, claiming the Huasen has discovered the Keystone.
Ji-Hun, the Doctor, and Yaz board Ching's ship. Ji-Hun finds out Ying Ki is his most trusted crew member Lee's descendant and reveals he gave the Keystone to Lee, who escaped his ship. The Doctor discovers Ying Wai's plan. Marsissus seizes the Keystone just as the Doctor realises the Sea Devils plan to use it to flood Earth.
The protagonists fight the Sea Devils aboard Ching's ship. The Doctor tampers with Ji-Hun's ship’s alien technology but destroying it requires two cables to be held together. The Doctor volunteers to stay back while the others escape, but Ji-Hun offers to sacrifice himself instead.
Madame Ching announces she has adopted Ying Ki and recovered enough treasure to pay the ransom. Dan calls his love interest Diane. Yaz confesses her romantic feelings toward the Doctor, who reluctantly turns her down out of fear of being hurt while affirming how special Yaz is to her and wishes they will stay together forever.