Coming into Doctor Who knowing that there's only one more episode until regeneration is always a surreal experience. Even though the franchise has promised the BBC centennial special will be feature-length, providing more than the usual 45-60 minute runtime to wrap up all the loose ends, it's hard to shake the awareness this is a penultimate episode of sorts. It's easier when this moment comes either towards the end of a season-long arc, with regeneration either as a season finale (like the Ninth Doctor) or buttoned as a Christmas special (as with the Eleventh and Twelfth).
The Doctor: You really pimped his ride! Do people still say that?
Yaz: It's 1807.
But it's much more challenging when the ride ends in a series of unconnected specials, as David Tennant's Tenth Doctor did at the end of the first Russell T. Davies era. The series tries for a stand-alone adventure doubling as a penultimate entry, asking for 45-minutes of television to serve two masters.
In Tennant's case, "Waters of Mars" was a brilliant stand-alone that wrapped up nothing. Perhaps that's what "Legend of the Sea Devils" should have done, especially as the episode was clearly meant to be a comedic romp that drew heavily from Pirates of the Caribbean. But too much angst and not enough fun make this as unsatisfying as the last time the show did a Spring Holiday special with 2009's "Planet of the Dead."
The Sea Devils — monsters that appeared from time to time in the Whoniverse between the Third and Fifth Doctor's runs — are released when legendary real-life pirate Madame Ching (Crystal Yu) chops down the statue that imprisons them, accidentally causing the death of the statue's guardian, Ying Was ( David Tse) in the process. The Doctor, Dan, and Yaz capture the Chief (Craig Els) in a sequence that felt like it was pulled directly from a Warner Bros. cartoon, but only for a moment before the Flying Magical Green Gemmed Pirate Ship with his buddies pops by to pick him up. (I assume he summoned them via the Ubyarrrrrrr app.)
Madame Ching's statue chopping was to aid in her search for the treasure of the Flor de la Mar, which was last seen in the possession of Sin Ji-Hun (Arthur Lee). Ying Was is the latest in a long line of Guardians of the Statue, and his son, Ying Ki (Marlow Chan-Reeves), takes up the mantle, with kindhearted Dan teaming up with him to chase down Madame Ching's ship. They're caught immediately, but Madame Ching's ire with them is short-lived because she's short-handed. Guo Podai and the Black Flag fleet are holding her crew hostage along with her toddler-aged sons; her treasure hunt is not about pirate booty but money to pay the ransom.
The Doctor and Yaz head back to the TARDIS, just as the Chief Sea Devil releases a sea dragon-like monster, a Hua-Shen. They travel to 1533 and find the treasure, hoping it will bring Madame Ching to their side. Their chase leads to the ocean floor, one of the series's more remarkable moments in its 59-year history. The TARDIS opens underwater (encased in an oxygen bubble, natch), and The Doctor and Yaz gaze upon the wonders of the other final frontier humanity has yet to explore properly, just as the Hua-Shen shows up and goes all Jonah and the Whale.
The Hua-Shen brings the Doctor and Yaz to the Chief Sea Devil, who reveals the Flying Magical Green Gemmed Pirate Ship is Ji-Hun's ship, and he's been trapped with them for ~175 years. Ji-Hun was responsible for initially trapping the Sea Devils, using a piece of the Flor de la Mar, the "Keystone," a gem of unlimited power. Said Keystone is the necklace passed to Ying Ki when he became the guardian . The Doctor commandeers the Sea Devils' ship, surfacing next to Madame Ching and bringing them all together to stop the Sea Devils from flipping the Earth's magnetic poles and turning the planet into Waterworld.
Once again, Dan's comedic chops are the funniest parts of the episode, from his terrible pirate costume to his repeated snarky asides. (He also gets to fight again, though with a sword, not a frying pan.) Ji-Hun is given the noble sacrifice to save the planet since he's a Man Out of Time. (He also kills the Chief Sea Devil, which the Doctor disapproves of, but the show doesn't dwell on it.) Before he dies, he bequeaths the treasure to Madame Ching so she can ransom her crew. She also adopts Yang Ki since he no longer has a statue to guard or a village since she was responsible for the Chief Sea Devil's release, leading to him slaughtering everyone.
Unfortunately, putting the Doctor and Yaz together is an excuse to circle back to the underbaked "Yaz is in love with the Doctor" plot, which thus far has consisted primarily of longing looks. The Doctor at least remembers River Song exists, mentioning their marriage. However, her attempts to talk up how Yaz is "one of the greatest people I've ever known" and how central she's been to their adventures only remind viewers how little the series has shown that rather than told it. But it's all very obviously a setup for the finale, with the Doctor's wish that "this could go on forever," a roundabout promise that it's all about to end.
With a trailer at the end of the episode that looks like the show is bringing back Tegan and Ace as part of the Centenary celebrations to link directly back to Classic Doctor Who, it's doubtful Yaz and the Doctor's ending will be little more than the same sort of long glances and words unsaid. (That being said, I would take a team-up of every living companion as part of next year's 60th Anniversary.) The show as least gives Dan more closure with Di than the open-ended dumping he received, letting him call home, so she can tell him how much she misses him, dropping broad hints he'll be home soon. L'Stav Haba'ah B'Liverpool, I guess.