Marco and Star's art class lets out, and they are about to leave for their next class when Marco notices a paint stain on Star's forehead. However, Star explains that it is not a paint stain but a heart spot, one of the first symptoms of Mewberty. Marco at first sees it as no big deal since everyone on Earth goes through a similar phase in their lives but Star tells him it is far more different than Earth puberty and could potentially destroy the school. Marco decides to take Star home, but already she begins to show an urging attraction to all the boys she passes and sprouts more hearts all over her body. She also develops the ability to creating purple sticky webs from her hands. Worried that she might harm Marco, she locks herself inside a school locker. Marco suggests checking Star's magic book and see if there is a way to stop her Mewberty, but Star says that she lost it in a bet with Ferguson.
Marco finds Ferguson in Spanish Class, and when he asks for Star's book, Ferguson tells him he gave the book to Janna. Marco soon finds Janna with the magic book and begins to search for a page on Mewberty but finds a little guy named Glossaryck instead. At first, he refuses to help Marco as he only serves princesses, but reluctantly volunteers to help him if he can provide him with some pudding. Meanwhile, Star is still hiding in the locker and manages to peal all the hearts off her body but more soon grow back when students pass by her locker. Her skin turns purple, her eyes begin to glow and now has an uncontrollable passion for boys. The locker she is in soon begins to overfill with hearts and form a heart-shaped cocoon. Eventually, Star emerges from the cocoon with six arms and wings.
While feeding the Glossaryck pudding, Marco asks him how he can stop Star's Mewberty. Glossaryck tells him it is a natural process that cannot be stop, but he assures that she may (or may not) return to normal. Suddenly, Marco hears a scream and goes off to check it out. The weather turns dual when Marco discovers half the school overgrown in purple hearts and vines. He also finds a number of boys trapped in purple webbing and soon finds Star about to wrap another boy in her web. Marco tries to reason with her, but Star ignores him and continues to wrap up her new captive. When he tries to interfere, Star reacts by wrapping him in her web until she hears Oskar's music. She attempts to wrap him in her web as well until Marco catches her in a tennis net. Marco holds tight to the net as Star tries to break free and begins to drag Marco all around the school. Glossaryck tells Marco that he is only making things worse by interfering. Finally realizing there is nothing he can do, he reluctantly lets Star go, who grabs Oskar and carries him up into the clouds. For a moment it all goes quiet and Star does not reappear. Just as Marco believes his friend is gone forever, hearts rain out of the sky, and Oskar returns to the ground unharmed. Then much to Marco's joy, Star returns, back to normal and with small wings.
- Eden Sher as Star Butterfly
- Adam McArthur as Marco Diaz
- Jeffrey Tambor as Glossaryck
- Jon Heder as Oskar Greason
- Abby Elliott as Janna Ordonia
- Nate Torrence as Ferguson O'durguson
- The title is a pun of the word puberty.
- Star's magic book appears for the second time in this episode, the first episode was in "Monster Arm".
- Starting from this episode onward, Mercury Filmworks no longer animates for the series.
- Star gets wings.
- It is revealed that Ferguson can speak Spanish.
- June 21, 2015 (Canada)
- August 4, 2015 (Germany)
- August 11, 2015 (Australia)
- September 29, 2015 (Latin America)
- October 10, 2015 (Turkey)
- October 23, 2015 (Israel)
- October 24, 2015 (Poland)
- November 7, 2015 (Spain)
- December 13, 2015 (Southeast Asia)
- February 5, 2016 (Japan)