As an anniversary gift for Marco's parents, Star invites them and Marco to Mewni, but must avoid being seen by her father as Star is not allowed to leave Earth. While they visit a Mewni village, Star sees her father strolling about, and directs the Diaz family into a deadly forest.

Star experiences "mewberty", that is, her skin breaks out in hearts whenever she is near boys. Although Marco thinks it's much like the Earthling condition, Star's is more extreme. She sprouts hearts all over her body, and transforms into a butterfly creature that captures boys and seals them away. Marco turns to the Glossaryck, a genie inside Star's book of magic, for advice on how to stop her.