Ex-pirate Kwazii, along with Captain Barnacles, medic Peso Penguin and the other Octonauts, embark on several pirate adventures while exploring and protecting marine life and habitats.

Join the Octonauts as they explore hidden depths of the Midnight Zone! Peso must conquer his fears to treat a Vampire Squid! Dashi and Inkling go head to head with hungry Gulper Eels! Then, Barnacles, Kwazii and crew must rescue sea creatures before an underwater volcano erupts!

When Saoirse and Ben's mother mysteriously disappears into the ocean, the two children go on an epic journey to find out the truth about her, and in the process, discover mystical secrets about both their mother and Saoirse herself. The film takes inspiration from the mythological Selkies of Irish folklore, who live as seals in the sea but become humans on land.