The Doctor, Kaz and Dan end up in 19th century China where they encounter one of the Timelord's old nemeses, a 'Sea Devil' (last encountered in the silly Warriors of the Deep (1984)), who is searching for a key to unlock the power to inundate Earth and return it to the rule of his aquatic brethren. The story doesn't make a lot of sense and is marred by one of those 'futz with the alien technology and the existential threat is conveniently nullified' cop-out resolutions. There is a nicely rendered but under-utilised sea monster, some unfunny pirate-themed humour, a poorly choreographed fight scene, and some more 'historic characters' who look like millennials playing dress-up. Kaz's romantic pining for the Doctor is revisited but fortunately she is rebuffed (surely the Doctor is lying when she says that Kaz is one of the most interesting people she's ever met). The Doctors' personal lives are rarely explored but it is canonical that the first Doctor had a granddaughter, so there must be some history of romantic attachments. The Sea Devils are an interesting homage to the originals and look pretty good (although sometimes a bit awkward and 'rubbery'). The story jumps erratically at times as if segments had been cut out (notably when the Doctor et al and a seemingly spontaneously freed Ji-Hun appear on the Flor de la Mar, and for either budget or covid reasons, the sailing ships seem to operate without crews, which just seems ridiculous. Overall, a simplistic story-line with uninteresting characters, unimpressive CGI and a contrived ending. The show's recent trend towards self-righteous posturing is kept to a minimum, so the episode is just weak, not irritating.