Two fraternal twins, Dipper and Mabel Pines, arrive at the remote town of Gravity Falls to spend their summer break with their great uncle Stan, but a mysterious journal hidden in the forest found by Dipper reveals the town's strange and uncanny secrets to the duo, and Dipper tries to save Mabel from her strange new boyfriend who appears to be a zombie, but is actually something much stranger. ("Tourist Trapped")
Dipper, Mabel and Soos embark on a boating expedition to prove that a sea monster exists beneath, hoping to win a thousand dollars. Meanwhile, Stan, after being abandoned by Dipper and Mabel, tries to find fishing partners. ("The Legend of the Gobblewonker")
Mabel sculpts a life-sized wax sculpture of Stan to add to his collection of wax museum figures. Sometime later, the wax figure's head goes missing, and Dipper is unable to discover the truth, finding a suspicion about Toby Determined, but it turns out to be incorrect, causing them to be laughed at by the local police. They discover that the wax figures—led by Wax Sherlock Holmes—attempted to murder Stan, but miscalculated, killing Wax Stan. They were frustrated with being left behind and nobody caring for him. They attempt to kill Mabel and Dipper, but using wax candles and sunlight, they melt them. In a mid-credits scene, Wax Larry King escapes death. ("Headhunters")
Dipper and Mabel meet an intriguing new neighbor: a psychotic child fraud named Li'l Gideon. When Gideon begins obsessing over Mabel and tries to win her over, Dipper and Stan are not happy, but Stan agrees to support it to get more money from a merger between Gideon's company and The Mystery Shack. When Dipper tries to call it off on Mabel's behalf, Gideon attempts to kill him, but is stopped by Mabel, who breaks up with him. In the aftermath, Gideon makes Stan his enemy again, forcing his father Bud to tear up the merger contract. ("The Hand That Rocks The Mabel")
Dipper attempts to woo Wendy by lying about his age. Hanging out with Wendy and her friends, the kids end up in an old, abandoned shop that is seemingly haunted. They discover that the ghosts died in the 1990s as a result of the then-developing rap music and are haunting the shop to get revenge against the teenagers. To stop them, Dipper tells them the truth about his age, inadvertently revealing the truth to Wendy, and dances the embarrassing lamby lamb dance, embarrassing himself. Wendy, however, thinks he is still cool after the incident and covers up the truth to her friends. ("The Inconveniencing")
When Dipper wants to become more "manly", he wanders into the forest on an epic quest and meets a strange species that teach him the ways of their masculinity. The species wants him to kill the MultiBear, it's archnemesis, but after discovering that it is a fan of the same pop music group that Dipper is, he spares it. Meanwhile, Mabel tries to get Stan to impress Lazy Susan. Mabel is successful. ("Dipper vs. Manliness")
Stan throws a party at the Mystery Shack; while Dipper produces clones of himself to help him impress Wendy and Mabel faces off against her new rival Pacifica Northwest in a karaoke battle. Dipper's clones get obsessive and lock him in a closet after he slowly begins to abandon the plan to impress Wendy, but he destroys all of them but one with liquid. He and the other clone talk to each other, before the clone dies by drinking soda. Meanwhile, Mabel befriends two girls, Candy Chiu and Grenda Grendinator, and defeats Pacifica in a karaoke battle. ("Double Dipper")
On the annual Pioneer Day at Gravity Falls, the kids discover that the town's founder was a fraud and a hoax, so they set out to expose the truth. However, when they do, Sheriff Blubs and Deputy Durland go after them to stop the secret from being revealed. They discover that Quentin Tremblay was the town's founder and that he was the eighth and a half president of the United States, but after he covered himself in peanut brittle, him being founder of Gravity Falls and president, was covered up. They send them to Washington D.C., but Tremblay breaks free and helps them escape by sending Blubs and Durland on vacation due to him never leaving office. Tremblay then leaves Dipper in charge and leaves to find his own journey. ("Irrational Treasure")
Dipper's wish comes true when he steals a time traveler, Blendin Blandin's time machine to undo a mistake and win Wendy over, while Mabel accompanies him to keep on winning her pig, Waddles, until when Dipper finally undoes his mistake, Mabel loses her pig to her nemesis, Pacifica Northwest. Mabel and Dipper fight over the decision, and realizing Dipper will not be happy without Waddles, undos the decision after returning from the past. Blandin is arrested by the time police after being blamed for time being altered. Elsewhere, Stan rigs a dunk tank to scam the town. ("The Time Traveler's Pig")
Dipper brings a video game character, Rumble McSkirmish, to life so he can fight Robbie, but when things go too far Dipper realises he needs to save Robbie. Meanwhile, Mabel tries to end Stan's fear of heights. ("Fight Fighters")
Feeling insecure that Mabel is becoming the alpha twin, Dipper seeks a magical crystal to grow taller. But when Gideon finds the crystal, he uses it to shrink the kids and tries to use it on Stan to take over the Mystery Shack. ("Little Dipper")
The people of Gravity Falls love Halloween so much that they also celebrate it in summer, with jack-o'-melons and much trick-or-treating. Dipper and Mabel are excited to join in on the fun, but when Wendy casually mentions that trick-or-treating is for kids, Dipper's outlook on the evening changes. The night gets even more complicated when a monster that Dipper accidentally insulted makes them fulfill his candy quota by the night's end, or else he will eat them. ("Summerween")
Mabel bets Grunkle Stan that she will make more money at the Mystery Shack by treating people nicely than he does by being rude and cranky. He heads off to win a fortune on a TV game show, while she deals with lazy employees and a real monster captured by Dipper. ("Boss Mabel")
As the kids, Stan and Soos fall into a seemingly bottomless pit, they tell each other stories to pass time. ("Bottomless Pit!")
Mabel befriends a cute merman trapped in the deep end of the public pool, and is determined to return him to his family in the ocean—even though that means her whirlwind of a romance will come to an end. Meanwhile, Dipper learns that Wendy is a lifeguard at the pool, so he decides that the best way to spend time with her is to volunteer. Elsewhere, it's all out war for the best lawn chair at the pool between Stan and Gideon. ("The Deep End")
Soos discovers a hidden room inside the Mystery Shack, and Dipper is happy to claim it as his own independent space, but there is one problem—Mabel wants the room too. They both compete for Stan's approval to win the room as part of a contest that gets even trickier when the siblings switch bodies due to the room's mysterious shag rug. ("Carpet Diem")
When Mabel discovers that her favorite boy band is actually a bunch of imprisoned clones, she sets them free, only to decide on keeping them for herself. Meanwhile, Dipper thinks that Robbie has brainwashed Wendy using a secret message hidden in the lyrics of one of his songs. ("Boyz Crazy")
A prehistoric pterosaur snatches Waddles after Grunkle Stan throws him outside, and Dipper, Mabel, Stan, Soos and Old Man McGucket go after the beast in order to save Waddles, while also working out friendship issues between each other. ("Land Before Swine")
The kids and Soos travel through Grunkle Stan's mind to defeat an ancient demon named Bill Cipher, summoned by Gideon to retrieve the code to Stan's safe in a ploy to get the Mystery Shack. ("Dreamscaperers")
After Gideon swindles the Mystery Shack away from Stan, everything in Gravity Falls appears to be even more west of weird—Dipper, Mabel and Grunkle Stan are forced to move in with Soos and his grandmother, while the town remains enamored with Gideon's charm. Meanwhile, Grunkle Stan isn't certain he can take care of the twins, and is considering sending them back home. Thus, the kids get on a bus to leave Gravity Falls; but a giant robot with Gideon inside starts chasing after them because he thinks Dipper has Journal 1. However, the twins defeat Gideon and win back their home. Grunkle Stan exposes him as a fraud by showing everyone the surveillance cameras hidden inside Gideon's pins, which causes him to go to jail and the townspeople to turn against him. At the end of the episode, Grunkle Stan has started the activating sequence for the portal in the shack basement after he collected all 3 journals. ("Gideon Rises")