"Butterfly Trap" is the first segment of the fifty-first episode of Star vs. the Forces of Evil. It premiered on March 10, 2018 alongside "Ludo, Where Art Thou?", and is the first segment of the sixteenth episode in the third season.
Queen Moon and the Magic High Commission prepare for Eclipsa's trial. Eclipsa herself enters casually as she wants answers of her own. Hekapoo is irritated that Star is there, but Moon allows her to as the trial was her idea. Sean, the Commission's sole security guard, is also present as a typist. Things get started roughly when Moon presents the "evidence" of Eclipsa's supposed villainy: stacks and stacks of papers and books that consist of "second-hand accounts, oral histories, folk stories, none of [which are] verified." As the entire group resign themselves to looking through them all, Eclipsa offers another solution, "Trial by Box". The Box of Truth is revealed to have been the Truth or Punishment box from Star's sleep over. With its setting changed to "Trial" and everyone swearing to tell the truth, the actual trial begins.
Omnitraxus Prime asks if Eclipsa wrote the "evil chapter" in the Book of Spells. Eclipsa confirms this, but adds that they were the ones who named it the evil chapter. Hekapoo asks if she left her husband to elope with a monster, she confirms this too. Rhombulus wastes his question on her having eaten any babies. Hekapoo reminds him that it was Bobipsa, and Eclipsa concurs though she does admit to hurting teenagers psychologically. All three answers results in Hekapoo checking her off as "evil" much to Star's annoyance. Moon asks if Eclipsa is sorry for any of the things she has done and she tells her no. The High Commission already announce her guilty, but Moon allows for her to ask a question herself. Eclipsa asks what they did to her daughter and the Commission deny anything, resulting in the Box calling them liars.
The Box proceeds to encase everyone and shrinks. The Commission refuses to answer with Star accusing them of being negligent in their own abilities. Rhombulus breaks down and admits that they got rid of her. Omnitraxus admits that King Shastacan, Eclipsa's husband, wanted nothing to do with Meteora so they got rid of her and replaced her with a peasant girl instead. Moon and Star reveal that they set up the whole trial to get the Commission to confess their deed with Star realizing that she and Moon are not royal by blood and therefore not related to Eclipsa. She solemnly releases her and she runs in sympathy.
- Eden Sher as Star Butterfly
- Grey DeLisle as Moon Butterfly
- Esmé Bianco as Eclipsa Butterfly
- Zosia Mamet as Hekapoo
- Carl Weathers as Omnitraxus Prime
- Kevin Michael Richardson as Rhombulus
- Josh Sussman as Sean
- Sean Schemmel as Box of Truth
- WatchMojo.com named this episode #9 in their "Top 10 Cartoon TV Plot Twists You Didn’t See Coming" list.
- Rhombulus accidentally admits that he has a crush on Queen Moon.
- It is revealed that Queen Festivia was originally a peasant girl before being transplanted onto the Mewni throne by the Magic High Commission. This means all of Festivia's descendants, including Moon and Star, are not true-blooded members of the Butterfly family.
- It is revealed that the game, “Truth of Punishment”, from “Sleepover“ is actually called the “Box of Truth”. It has numerous settings which include Party, Intervention, Bachelorette and Trial.