DC Super Hero Girls: Legends of Atlantis (not rated)
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Caution: spoilers!
This is either the third or the fifth direct-to-video movie in the DC Super Hero Girls franchise, depending on whether or not you include the Lego DC Super Hero Girls movies. (I sort of don't. I haven't seen them as yet, but I don't discount the possibility that I might want to see them at some point. Whether or not I'd consider them canon is another matter.) It came out in October 2018, in the middle of season 5 of the webseries, although the timing is a bit confusing. It seems to take place both before Mera came to Super Hero High (early in season 4) and after Raven came to the school (late in season 4). Oh, and this is a minor point, but you can see in the background that Hal Jordan is still the Green Lantern at SHH, though he left at the start of season 4. (I also thought, while watching the movie, that details of how Mera came to SHH were changed. But then I rewatched her first episodes, and was surprised to see there actually weren't any details of how she came to the school. So I guess, at least on that point, the movie doesn't contradict the webseries. But the timing is still weird.)
Anyway, it begins with a flashback to when Wonder Woman was a little girl in Themyscira, playing on the beach with her pet kangaroo, Jumpa. Nearby, under the ocean, there are a pair of sisters named Mera and Siren, who are looking for a home. They plan to find Atlantis, obtain the king's trident, and... well, I'm not entirely sure what the whole plan is. I just know Siren is obviously manipulating Mera, though Mera seems oblivious to it. At one point, Siren stops at a spot on the ocean floor where she uses a spellbook, the Book of Legends (which looks to me kind of like the Necronomicon), to open a trench and release some mer-monsters (which are called Trench). Mera definitely wasn't expecting that, but it doesn't seem to faze her as much as it should. (Incidentally, the Book of Legends had previously been seen at the start of season 5 of the webseries, so that season is definitely set after this movie.) Siren also creates a tidal wave, which threatens Themyscira. Mera tries to stop it, but when Wonder Woman sees her riding the wave, she assumes Mera is controlling it. She tries to stop Mera and Siren, but nearly drowns. However, Siren ends up losing the spellbook, which falls into the trench, and is followed by the Trench, and the trench recloses. Wonder Woman later winds up on the beach, where her mother finds her.
The story then flashes forward to the present. (At first I assumed somehow this incident from Wondy's childhood would come up in conversation, and Mera would realize she and Wondy had met before, and things would get all awkward between them. But as I said earlier, this takes place before Mera came to Super Hero High.) There's a show-and-tell kind of thing going on in one of the classes at SHH, and Hawkgirl presents the Book of Legends, not knowing exactly what it is. (She says she found it during an "archaeology abroad tour," though I still found it highly improbable that anyone would ever have found it.) But Wonder Woman and Raven both recognize it, and warn everyone how dangerous it is. So Batgirl makes a high-tech safe to keep it in, and asks Raven to add magical protection to the safe. Unfortunately, her spell goes awry, and ends up switching Batgirl and Supergirl's powers. Also, far away under the ocean, Siren senses that the Book of Legends has been activated. She and Mera are still homeless, all these years later, and she convinces Mera to help her obtain the book from Super Hero High. There is subsequently a battle, in which both Mera and the superheroes each believe the other to be the bad guys. Siren finally gets the book, and she and Mera then go back to the trench to release the Trench again.
Katana, Wonder Woman, Bumblebee, Frost, Beast Boy, and Flash travel on Katana's boat to search for Siren and Mera, while Supergirl and Batgirl travel by sub to search for Atlantis, and warn the king. After the heroes on the boat stave off an attack by Trench, Bumblebee dons a diving suit to follow Mera and Siren. She watches as they attack Atlantis, and Siren steals the trident from the king, Aquaman. Siren then puts him into a permanent sleep, and also imprisons Mera, whom she doesn't trust because Mera doesn't want surface dwellers to be hurt. (There are also hints that Mera could develop a crush on Aquaman, though that's not important to the plot; it just seems to be thrown in as a teaser for comic fans.) Meanwhile, we see occasional scenes back at SHH, where Harley is trying to get Raven to come out of her shell and open up to the possibility of friendship. (There's also a scene where Harley plays psychiatrist, which I quite liked, because it's gotta be another teaser for comic fans.)
Bumblebee hears Siren say she plans to sink Metropolis, to eliminate all the heroes at SHH so they won't be able to stop her from world conquest. So she warns her friends about that, and also frees Mera to help them against Siren. Harley and Raven join their friends under the ocean, while all the other heroes at SHH have to protect Metropolis's citizens from the rising waters, and attacking Trench. So... there's plenty of action on two fronts. Of course Siren is ultimately defeated, and Raven manages to wake up Aquaman, and Mera returns his trident to him. And of course Batgirl and Supergirl get their respective powers switched back. And the Book is later stored away in a vault at SHH, in a scene that reminded me of "Raiders of the Lost Ark." (It's supposed to be somewhere no one will ever find it, which is kind of funny, considering how easily it's found there by students at the start of season 5.) And Mera begins attending SHH.
So... it seems like I've spoiled pretty much everything. But I've actually left out a lot of fun details. There really is a lot of humor throughout the movie, in addition to all the drama and action.
The 2016 TV special Super Hero High is included as a bonus feature on the DVD.
