DC Super Hero Girls: Legends of Atlantis (2018) on Netflix

Plot summary

Siren steals the Book of Legends from Super Hero High and sets out to locate the Trident of Atlantis, hoping to rule the ocean. Once the book vanishes, Wonder Woman, Batgirl, Supergirl and Bumblebee lead the Super Crew away from school and into the sea. Their search brings them to Atlantis, where they meet Mera and Siren. The two ocean dwellers are a difficult match for the young heroes, who need to work as a team if they are to recover the stolen book. Meanwhile, after an accident involving Raven, Batgirl and Supergirl end up with each other's powers. Harley Quinn tries to help Raven gain control of her magic while the others face the underwater threat.