Steven befriends a magical mirror that can mysteriously communicate with only him and ends up setting free its trapped inhabitant, a Gem named Lapis Lazuli.

Lapis Lazuli disappears after reigniting a past conflict with Garnet, Amethyst and Pearl, and takes the ocean along with her, leaving Beach City in a panic on the first day of summer. Steven, Connie, Greg, Lion and the Gems go on a mission to confront Lapis and set things back to normal.