Moon reveals that Mina is acting on her orders just as the rooster crows. Star and her sisters try to cheer her up, but she runs away with them giving chase. Ludo's minions Spider and Eagle volunteer by venturing into each portal, but to no success in finding him. While King River returns on some eagles, Queen Moon checks on Eclipsa who is still encased in crystal, however as Moon leaves, the crystal that holds Eclipsa cracks. During the event, they meet fellow shoppers who will stop at nothing to get the best knight gear. Pricing, channels, features, content, and compatible devices subject to change. However, their friendly talk is interrupted by Moon and the Magic High Commission, who capture Eclipsa.
Star eventually finds Globgor running away from Mewni, believing his presence will only endanger his family. The Forces of Evil will follow teen princess from another dimension Star Butterfly and her best friend Marco Diaz as they continue their adventures in and explore new dimensions. Marco tricks Star into thinking the sayings in fortune cookies can really predict the future. They end up having a hard time when they must deal with the various jobs of the goblin vendor Roy and the many lines that lead to the Goblin Dog truck. He meets with some of the court entertainers, and they steal the key, but River refuses to escape.
Star talks to Ponyhead, Janna, Kelly and Starfan13, and they convince her to ditch the party and instead attend another party, where she meets Oskar. This angers the Stump, and it captures her friends and her parents. Ben then reveals that he actually locked them inside in hopes this would happen, having deduced they had feelings for each other. Janna goes missing and screams attract the kids back to the house. Marco admits he likes Jackie, but the cube insists someone has lied, and informs that Marco wasn't the one who lied. Upon retrieving her ledger, Moon finds that what has been haunting the castle all along was actually Mina Loveberry.
Afterwards, Eclipsa reveals she has been searching for pieces of the Book of Spells to restore it and use the knowledge inside to help the kingdom. He competes with a giant spider as they go. Ludo and his minions attack and hijack the bus. Star and Janna try to find replacement batteries by sneaking out to Oskar and getting ones from his keytar. The Naysaya can only go away when it is appeased, so Marco tries giving it what it wants, which is some cereal, but it doesn't go away. Tom explains that she is scared that Star will hurt Tom again and that she knows that Star kissed Marco.
Star finds herself in a sea of dark magic inside Ludo's body, and Ludo is taken over by Toffee again. Star follows Mina and tries to train in her ways, which annoys Marco. Unfortunately for Ludo, this involves reuniting him with his former henchmen and rebuilding his old castle. Star allies with Buff Frog and the kicked-out Ludo to storm the castle and beat up Ludo's former minions and free Marco. They arrive at the monster temple, where they find Eclipsa's monster husband, Globgor, who has also been crystallized. Later Janna burns the Stump on the fireplace.
At the end Tom's mom takes the last photo of Star and Tom for the photo album and explains that the rain appeared because of her emotions. Afterwards, Marco finally proves to Mariposa that they are siblings and that he loves her. However, the arm harbors an evil sentience that intends to take over the world. Rhombulus is arrested while Moon, who is amongst the attendees, tells the Mewmans that they need to stop following her and think for themselves. Star must solve the problem or her world will fall apart. Missing her family and friends, she returns to a very worried Marco and turns herself in to the police who have her wash the other squad cars as part of her police-supervised community service. Star and Buff Frog realize there may still be hope into proper integration.
Candle but is captured by Tom, and must duel Tom to get his freedom. Star, River and Marco, meanwhile, have been searching for Moon, but to no avail. Rhombulus is arrested while Moon, who is amongst the attendees, tells the Mewmans that they need to stop following her and think for themselves. Star takes the concept of going to battle literally, and booby traps the field with bombs, evil kittens, and giant birds. Star returns to the Diaz family where she meets Marco's newborn younger sister Mariposa.
The other monsters are summoned for a fight but they are beaten up. Marco steps inside and tries to casually tell Tad that Kelly wants to move on, but Tad then easily guesses that Marco himself is miserable because of hidden feelings for his crush on Star. To ensure that she tells the truth, Moon brings in a game of truth or pain. Star and the king then go through several rituals that Star rushes causing them to have to start over. After attempting to do the same by dancing with her other male dance partners, she then eventually confronts him alone to ask him why.