Sweetheart is first mentioned as part of the main quest to help cheer Captain Spaceboy up from his love sickness. There, Rosa, a fan of Sweetheart, is met for the first time. Sweetheart is also the one whom Captain Spaceboy is lovesick about.
Before the castle
After overcoming his fear of spiders, Omori and his party can access Pyrefly Forest. There, the friends meet Mari, who reassures them by saying that everything will be fine, and not to be afraid of the spiders, primarily to Hero. Just a second after, the protagonists encounter King Crawler, an enemy made up of multiple undead Sprout Moles. After defeating him, they can enter the Sprout Mole Village. Omori and his friends find a colorful door, which leads to Sweetheart's show.
In the Sprout Mole Village, there are several doors or stairs that lead to the various residences for the Sprout Moles. In addition to the houses, there is also a large pink door which leads to a theatre.
In the theatre, the friends watch Sweetheart live on stage, looking for a new boyfriend despite her very recent breakup with Captain Spaceboy. Unfortunately, however, one of the three competitors is hit by an electric discharge, which turns out to come from the gym at the bottom of the Sprout Mole Village. After repairing the damage that caused the discharge, the Mustache Mole, whom was looking for a replacement for the third candidate set their eyes on HERO and his charm. The Mustache Mole has Hero follow him to the backstage where he ends up being a replacement for the third contestant. Sweetheart immediately sets her eyes on him and decides that she wants to marry him. Hero, however, reluctantly tells Sweetheart that he is not interested in being her husband. While at first, she couldn't possibly believe what he was saying was true, she then realizes that she is being rejected and angrily throws Omori and his friends into the castle's dungeons.
Inside the castle
In the dungeons, the protagonists face numerous enemies and puzzles, and eventually find themselves in Sweetheart's castle, pretending to be normal Sprout Moles. In this castle they must help the Sprout Moles perform Sweetheart's wedding tasks, such as conducting a choir or baking a cake. After leaving the castle and solving a puzzle, Omori's party finally returns to the theater, where they find Sweetheart, about to marry herself. Before performing this act she is stopped by Aubrey, who shows pity for her. Sweetheart becomes angry once again at Hero's rejection and begins to fight with Omori's party.
After unlocking the Underwater Highway, Sweetheart can be seen at the first toll station. Dialogue is exchanged between the characters and Sweetheart, where she tells of how she has divorced her husband, now titled Space Ex-Husband. She claims, finding a suitor was never going to work and that, with her rich status, she plans to have sea witches make a partner for her. After she is gone, Kel and Aubrey joke on how wherever Sweetheart is, there's some sort of disaster.