When Dipper wishes he could go back in time and undo a mistake he made with Wendy in which he accidentally hit her and she starts going out with Robbie, he discovers that there is a time machine that can help him go back in time. Meanwhile, it's love at first sight for Mabel when she wins a pet pig at the fair.
The episode begins with Stan proudly showing Mabel his Mystery Fair, stating that he "spared every expense." Dipper falls in a car of the sky tram next to them, and says that the sky tram is broken, as well as "most of his bones." Grunkle Stan laughs and tells them to put fake safety inspection certificates on attractions at the fair. Mabel asks Stan if it's legal to do that, and Stan replies that anything is legal when cops aren't present. He then checks on Soos to see how the dunk tank is coming along. He is satisfied with how rigged it is, despite Soos' suggestion that a laser arm cannon could actually bring it down. Stan mentions that his screwdriver is missing, and Soos suggests that something paranormal may have taken it. Stan tells him that he's been spending too much time with the twins, and leaves. Meanwhile, Blendin Blandin is seen in the foreground, using the screwdriver to fix his cloaking device.
Dipper and Wendy walk around the Mystery Fair until Wendy gets mustard on her shirt and has to go clean it off. Dipper runs into Mabel and they discuss Dipper's asking Wendy out. Robbie shows up and asks if they know where Wendy is, then leaves when Dipper refuses to tell him. Dipper decides that he needs to keep Robbie and Wendy apart. Mabel says she will help, but sees a sign to win a pig and leaves Dipper. She decides she will try to win "ol' fifteen poundy," because she thinks he may be saying her name (or doorbell). She guesses the pig's weight correctly, so the farmer asks her if she is a witch.
Dipper and Wendy walk around the fair until Wendy sees a stuffed creature of indeterminate species as a prize for winning a knock-over-the-milk-bottle game. She expresses a huge desire for the toy and Dipper attempts to win it for her. He hands the carnival attendant a ticket for one ball and the man reminds him he only has one chance. Wendy smiles and gives him a thumbs up. Dipper throws the ball and misses, hitting the crate the bottles are stationed on instead. The ball ricochets and hits Wendy in the face, giving her a black eye. Dipper assures Wendy that everything is going to be fine before running off to get her some ice.
He retrieves a bag of ice but crashes into Blendin Blandin on the way back, spilling the ice everywhere. Blendin runs off without apologizing and Dipper manages to gather up the ice just in time to see Robbie soothing Wendy's eye with a snow cone. Robbie then asks Wendy if she would like to go out with him. Wendy says, "Yeah, I guess so," the same words she used when she agreed to hang out at the fair with Dipper. Mabel, unaware of what just happened, runs up to Dipper and gushes about her new pig, Waddles. When she realizes he's not paying her any attention, she asks what's wrong and sees Wendy and Robbie run off to ride The Tunnel of Love and Corndogs.
Dipper spends the remainder of the fair heartbroken, staring up at the sky on a skee ball game. Mabel tries to use Waddles to cheer him up, but Dipper is much too busy dwelling on the fact that his one mistake caused him so much pain. He spots Blendin, on whom he now blames his misery, and accuses him of spying on them. After using advanced technology in a very feeble attempt to escape, Blendin confesses that he is a time traveler. Dipper sees Wendy and Robbie on the Ferris wheel and asks if he can borrow Blendin's time machine, which appears to be a simple tape measure. Blendin denies the request, claiming the device is far too complicated for them to use properly After the twins dismiss the machine, Blendin travels back in time twice, slightly aflame on the second return. He then explains that he is on a mission to prevent a series of time anomalies from happening at the fair, but for whatever reason he has been unable to find them. Mabel gives him some tickets for the fair, claiming that he deserves a break. He happily agrees, but swears that he'll be keeping a close eye on them and the machine.
Blendin then arrives at the barrel ride, where Soos says he has to take his tool belt off to prevent it from potentially flying off and accidentally fixing a ride. Blendin tells him to guard it and Soos promises he will watch the belt like a hawk while Blendin gets on the ride. Dipper steals the device while Soos is distracted by the ride and runs back to the Mystery Shack with Mabel and Waddles. The twins then travel 6 hours back in time so Dipper can undo his mistake with Wendy and Mabel can relive the best moment of her life (winning Waddles). Dipper finds Wendy and she again says that she wants one of the stuffed animals of indeterminate species. Dipper tries again but, due to time paradox, his actions involve her eye being injured yet again. Robbie still comes to her aid with the snow cone and everything happens the way it did last time, excluding Dipper's involvement with Blendin and Mabel causing an angry mob because she said she was a witch.
Mabel and Dipper keep repeating their actions, Mabel bonding deeper with Waddles and Dipper failing even harder every time. Finally, Dipper comes up with a long and complicated plan that requires Mabel's assistance. This time, Dipper intentionally aims for the tent's roof, where it eventually makes its way to the storm drain atop the Mystery Shack. Mabel moves the drain to that the ball flies back to the fair, hits the target on the dunk tank (and unsurprisingly still fails to dunk Grunkle Stan, much to the disappointment of the carnival goers), knocks the pizza out of the Pizza Guy's hands, removes the top of Robbie's snow cone, and successfully takes down the milk bottles without damaging Wendy's eye. The carnival attendant awards the stuffed animal of indeterminate species to Wendy, who gleefully shows it to Robbie while proclaiming Dipper won it for her. Robbie becomes flustered and pulls up his hood, walking away without asking Wendy out. Unaware of his intentions, Wendy shrugs and expresses her delight at being able to spend the day hanging out with Dipper.
Mabel, now having come down from the roof, is thanked by Dipper before she goes to win Waddles. However, Pacifica Northwest has already won Waddles, before Mabel can win him. Mabel screams continuously, until she finds Dipper disembarking the Tunnel of Love and Corndogs with Wendy. Mabel tells Dipper that she has lost Waddles to Pacifica and that they need to go back and fix the timeline. Dipper says that this timeline is the only one where he can avoid harming Wendy and they can't go hopping through time anymore. Upset, Mabel fights for control over the time machine and accidentally sends them back to the Oregon Trail. There, she intentionally messes with the people's knowledge of future inventions and advancements, such as braces, light up shoes, calculators, women's suffrage, and high-fives. Dipper then sends them to the Age of the Dinosaurs for a few seconds before Mabel takes them to the distant future (wherein the Time Baby is going on a destructive rampage). The pair then hop through some of the earlier events of the summer, leaving behind various items. The time machine starts shorting out while they run by a snow covered Mystery Shack, wherein it appears to be a young Ford. They disappear before he can see them, reappearing in the successful Dipper timeline. He tells Mabel that it's over; this timeline is the one they have to keep.
Dejected, Mabel begins thumping her head on a totem pole. Dipper says that she'll soon forget about Waddles, traveling further and further into the future to prove his point. When he goes a day, a week and a month into the future, he sees that her condition has worsened to the point where vines have begun growing over her disheveled body. She has now been turned into a tourist attraction for the Mystery Shack. He regrettably travels back to reinstate the original timeline, telling Wendy that people make mistakes and should always be forgiven before setting in motion the events that started it all. Wendy walks off with Robbie as Dipper looks on sadly. Mabel and Waddles then appear and express their gratitude by tackling him to the ground, Dipper admits that he couldn't break her heart and there was still time for him to win Wendy's heart later in the summer.
Blendin realizes what has happened and takes the time machine back seconds before the Time Paradox Avoidance Enforcement Squadron members Lolph and Dundgren appear and arrest him for violating the Time Traveler's Code of Conduct. Blendin then promises to mess up Dipper and Mabel's life by making their parents never meet.
Thanks to Soos' welding, Grunkle Stan has managed to avoid getting dunked at the dunk tank. He happily taunts one of the Time Paradox Avoidance Enforcement Squadron members, resulting in him firing "a futuristic laser arm cannon" at the target and successfully dunking Grunkle Stan. They disappear and Dipper and Mabel realize that they were the cause of all the time anomalies. Dipper groans as he sees Robbie and Wendy sharing a caramel apple on a stick. Mabel then sends Waddles (who had earlier expressed a love of caramel apples) off to the pair. Startled, Robbie drops the apple and spills a barrel of boiling hot water on his pants, causing them to shrink painfully. All the carnival goers laugh at his expense, including the still submerged Grunkle Stan.
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- When Blendin is enabling the camouflage setting on his suit, it shows a picture of dinosaurs stuck in tree sap from the "Land Before Swine," the arcade from "Fight Fighters," and Scuttlebutt Island from "The Legend of the Gobblewonker."
- One of the pioneers exclaims "By Trembley," referring to Quentin Trembley from "Irrational Treasure."
- The Time Baby is first mentioned in a document found in "Irrational Treasure" which states that he was originally frozen in a glacier in Antarctica.
- Dipper and Mabel leave behind several items in the past, forcing Blendin to remove them and thus explaining his appearance in "Tourist Trapped," "The Legend of the Gobblewonker," and "Headhunters."
- When Dipper and Mabel travel back in time to the late 1970s-early 1980s, Ford Pines, who makes his official debut in "Not What He Seems," looks out his door.
- Viewership: This episode was watched by 4.138 million viewers on its premiere night.
- In this episode, Mabel is wearing a flower on her head instead of a headband, and is wearing pink sneakers instead of her usual black shoes with white socks.
- Bill Cipher is seen on the playing cards that fly off the table when Dipper and Mabel go back in time.
- Dysentery is a sickness that spreads through water, hence the Oregon Trail traveler offering water and asking "Who wants dysentery?"
- This is the first time the Author appears onscreen.
- During the ending credits of this episode, there was a cryptogram that reads "MLG S.T. DVOOH ZKKILEVW." Once decoded, it reads "NOT H.G. WELLS APPROVED." This is because of H.G. Wells' famous book, The Time Machine.
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