Here's Starfleet under the sea:Lamia, a sixteen-year-old mermaid princess, is dissatisfied with life under the sea and curious about the human world. She constantly ignores the warnings of her father, King Dr Benn, and adviser, PPA, that contact between merpeople and humans is forbidden, longing to join the human world and become a human herself.One night, Lamia travels to the ocean surface to watch a celebration for the birthday of Prince Shiro, with whom Lamia falls in love. In an ensuing storm the ship is destroyed and Lamia saves the unconscious Shiro from drowning. Lamia sings to him, but quickly leaves as soon as he regains consciousness to avoid being discovered. Fascinated by the memory of her voice, Shiro vows to find who saved and sung to him and Lamia vows to find a way to join him and his world. She visits Makara the sea witch.Makara makes a deal with Lamia to transform her into a human for three days in exchange for Lamia's voice, which Makara puts in a nautilus shell. Within these three days, Lamia must receive the 'kiss of true love' from Shiro; otherwise, she will transform back into a mermaid and belong to Makara. Lamia is then given human legs and taken to the surface. Shiro finds Lamia on the beach and takes her to his castle, unaware that she is the one who had saved him earlier. Shiro's dad, Prof Hagen, urges him to forget his dream, and turn his attention to a real loving girl nearby, indicating Lamia. Makara takes the disguise of a beautiful young woman named "Keeli" and appears onshore singing with Lamia's voice. Shiro recognizes the song and, in her disguise, Makara casts a hypnotic enchantment on Shiro to make him forget about Lamia. Prof Hagen declares "This mystery maiden really exists, and she truly is lovely!"The next day, Lamia finds out that Shiro will be married to the disguised Makara. Kirara disrupts the wedding with the help of various animals. In the chaos, the nautilus shell around Makara's neck is broken, restoring Lamia's voice and breaking Makara's enchantment over Shiro. Realizing that Lamia was the girl who saved his life, Shiro rushes to kiss her, but the sun sets and Lamia transforms back into a mermaid. Makara reverts to her true form and kidnaps Lamia. Dr Benn confronts Makara and demands Lamia's release, but the deal is inviolable. At Makara's urging, he agrees to take Lamia's place as Makara's prisoner. Lamia is released as Dr Benn transforms into a polyp and loses his authority over Atlantica. Makara declares herself the new ruler and quickly grows to monstrous proportions.Lamia and Shiro reconcile on the surface just before Makara grows past and towers the two. She then gains full control of the entire ocean, creating a storm with a maelstrom and shipwrecks, one of which Shiro commandeers. As Makara attempts to destroy a trapped Lamia in the maelstrom, Shiro turns the wheel hard to port and runs Makara through the abdomen with the ship's splintered bowsprit, mortally wounding her. After her death, Makara's power breaks, causing Dr Benn and all the other polyps in Makara's garden to revert back into their original forms. Later, after seeing that Lamia truly loves Shiro, Dr Benn willingly changes her from a mermaid into a human. An unspecified amount of time later, Lamia and Shiro have their wedding on a ship and depart.