The fifth film in the Little Mermaid Series. It is set before Ariel was born. King Triton is young, has a small red beard, and he has green skin. His father, Poseidon, tells him, "When you grow up, your skin will become any colour you choose."
King Triton learns about humans. He goes to Olympus, and meets his cousin, Hercules. He also meets his other cousin, Athena. They fall in love. However, Athena is stolen by Morgula, the mother of Ursula and Morgana.
Morgula has purple skin, is tall and thin, and has white snakes for hair.
Triton must team up with Hercules to get it back. At the end, they rescue Athena. She asks Triton to marry her. He says he will. The next day, he becomes grown up, and chooses a pink, normal, HUMAN skin colour, to match Hercules's. He earns a trident called The Mystical Trident.
But then, his father Poseidon goes missing! Hercules tells Triton that he promises to do whatever he can to find Poseidon.
It is continued in The Little Mermaid VI: The Hero of Greece.