In China, 1807, a pirate queen called Madame Ching (Crystal Yu) raids a village and happens to release a Sea-Devil named Marsissus (Craig Els) from a stone statue. He kills Ying Wai (David K.S. Tse), father of Ying Ki (Marlowe Chan Reeves), and the one who was tasked with keeping the Sea-Devil contained. The Doctor (Jodie Whittaker), Yaz (Mandip Gill), and Dan (John Lewis) arrive to confront Marsissus. Ying Ki and Dan take a powerful gem and swim to Ching’s ship for revenge. Ching reveals that she is looking for the treasure of Ji-Hun, who vanished while looking for the Flor de la Mar treasure. The Sea-Devils release an enormous leviathan called the Huasen.
The Doctor and Yaz take the TARDIS back 274 years and see a Sea-Devil betraying Ji-Hun, then, returning to 1807, they search for his ship to no avail. The Huasen takes the TARDIS to the Sea-Devils’ undersea lair. They are looking for the Keystone gem. Pretending to have it, the Doctor and Yaz use it as leverage to board Ji-Hun’s ship, which uses alien technology. Ji-Hun himself is imprisoned in stasis. He admits to tricking the Sea-Devils. Marsissus calls the Doctor’s bluff, saying that the Huasen has found the Keystone.
Ji-Hun, the Doctor, and Yaz board Madame Ching’s ship. Ji-Hun discovers that Ying-Ki is the descendant of his trusted crewmember Lei Bao and reveals that he gave the Keystone to Lei, who escaped. Marsissus seizes the Keystone just as the Doctor realizes that the Sea-Devils plan to use the gem to flood Earth.
They battle the Sea-Devils on board Ching’s ship. The Doctor messes with the tech on Ji-Hun’s ship, but destroying it requires staying to hold two cables together and Ji-Hun offers to sacrifice himself. Madame Ching announces that she has enough treasure to pay the ransom and will make Ying-Ki a member of her crew. The Doctor takes Yaz and Dan ashore and the latter leaves a message for Diane. He is surprised when she returns his call. The Doctor wishes to spend more time with Yaz.
This Easter Special episode was directed by Haolu Wang and written by Chris Chibnall and Ella Road. The Sea-Devils had not been seen since Warriors of the Deep in 1984. It received a score of 76 on the Audience Appreciation Index but achieved low viewer numbers. It was only 47 minutes long, short for a Special. Craig Els, who played a Sea-Devil, was also the dog-faced alien Karvanista in the Flux series of Doctor Who. There was in fact a real female pirate queen named Madame Ching Shi, who lived from 1775 to 1844. She was active from 1801 to 1810 and was a real piece of work who killed many and commanded 70,000 men.