When Mabel befriends a cute merman trapped in the deep end of the public pool, she's determined to return him to his family in the ocean - even though that means her whirlwind 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 become the assistant lifeguard.
The episode opens with the exterior of the Mystery Shack, and with Toby Determined's voice on the radio describing the day as possibly the hottest day of the summer. Due to the extreme heat, a squirrel spontaneously bursts into flames out of nowhere, Wax Stan then starts to slowly melt, the fish tank in the living room begins to boil, the wallpaper peels off, and Dipper, Mabel, Grunkle Stan, and Soos lie around, too hot to do anything. After Waddles licked Stan's face, Stan, and Waddles had a pseudo-argument, Toby announces that the Gravity Falls Pool will be opening today. The four are excited to set out, though Stan is hindered because he is stuck to the floor. After the twins peel him off with spatulas, he rushes outside, where he catches on fire, and Mabel says hastily, "he'll be fine".
At the pool, the gang has random small talk before Mabel catches sight of Mermando and instantly develops a crush on him. When she inquires as to who he is, Soos tells her a rumor that he never leaves the pool and describes him as a "mysterious loner." She grows even more infatuated with him, much to the annoyance of Dipper and Stan, and follows Soos' instructions to go talk to him. As she runs to her believed true love, she crashes into various objects and irritates many people at the pool, and the boys discuss their opinions on Mabel's boy craziness. Suddenly, Stan is hit by a water balloon, and Wendy reveals that she is the one who threw it and has a job at the pool as a lifeguard. When Dipper learns that the pool is hiring assistants, he decides to apply for a job as an assistant lifeguard so he can spend quality time with Wendy, but he must first get the approval of the emotionally unstable Mr. Poolcheck.
Meanwhile, Mabel abruptly meets up with Mermando, and she promptly introduces herself and displays her highly outgoing and bouncy personality, which charms Mermando. When Mabel sees that he has taken a liking to her, she proceeds to ask him out on a date, but he replies that he cannot, due to a "terrible secret." This statement both upsets and intrigues Mabel. The scene then switches to Dipper talking to Mr. Poolcheck about the lifeguard job, and Poolcheck says he thinks Dipper may be fit, due to his SPF 100 sunscreen. However, Poolcheck also describes the pool as highly unruly and asks Dipper if he thinks he can handle it. After seeing Wendy, he replies that he thinks he can, and Mr. Poolcheck gives him the job.
Stan goes on to show Soos what he views as the perfect lawn chair, due to it being "equidistant from the snack bar and the bathroom, with just the right amount of sun and shade, and pointed away from where Old Man McGucket lotions himself." As he proceeds to sit down on it, he finds that Gideon Gleeful already has, solely due to the purpose of antagonizing Stan. When Stan tries to remove his young nemesis, Wendy charges him with breaking the pool rules and he is put in "pool jail." As he begs for his release, Dipper arrives at the site and tells Wendy that he will be his assistant, and the two gleefully run off to "abuse their power," despite Dipper's initial worries that Poolcheck may catch them. Back in the jail, Stan engages in conversation with the inmates.
Mabel then makes another attempt to bond with Mermando, and she gives him a wet sandwich to try to convince him to tell her his secret. Mermando is persuaded to tell her: he is a merman. After he tells Mabel his name, he divulges how he arrived at the pool. An unknown amount of time ago, he had been swimming with his friends, a pod of dolphins, in the Gulf of Mexico when he got caught in a fishing net. After being transported to Gravity Falls, Oregon, he entered the lake and tried to swim back to the ocean. However, he failed after being hit by a log, and he fell down into the forest. Had he not been licked by the deer, he would have died of dehydration, but soon fell down a pipe and into the pool. Mermando expresses thoughts that Mabel most likely finds it weird the he is not human, though Mabel tells him otherwise, and the two go play a game of Marco-Polo.
As Gideon puts sunscreen on himself, Stan, jealous and infuriated, peers at him from a distance, and gets the idea to blind Gideon by reflecting a beam of sunlight at him. His plan backfires when the boy anticipates this and reflects the light back off his goggle, hurting Stan's eyes and causing him to fall into the water. Dipper and Wendy then play a prank on Soos by impersonating a duck inner-tube, saying that the tubes have been enslaved and need him to liberate them. As Poolcheck announces that the pool is closing, Dipper and Wendy decide to hide from him, tells Soos to get away from the duck inner tubes, and instructs Dipper to lock up the pool supplies. Mabel then plans to sneak during the night to see Mermando.
After dark, Mabel arrives in the Mystery Cart, and she climbs over the fence to enter. In the pool, she shows the merman a scrapbook of human activities. When she shows him a picture of her family kickboxing, Mermando is saddened by the memory of the family from whom he was separated when he was abducted. When Mabel asks why he does not simply leave, he replies that it is extremely difficult for him and has not tried after his single failed attempt. He says that he is glad that he met Mabel there, however, and Mabel sees this as an opportunity to get her first kiss. But, when Mermando asks what she is doing, she blames it on sour candy and denies her friend's request for some.
The next day, Dipper and Wendy are seen playing ring toss by throwing life preservers at patrons. Then, Mr. Poolcheck confronts him about a broken pool skimmer. As Mabel tries to play ball with Mermando, he is too depressed to do so, as he still misses his family terribly. She is upset by seeing him so sad, so she agrees to help him escape. Later that night, while Dipper is on night patrol, he catches his uncle breaking in to claim his desired pool chair ahead of time, then possibly break some pool supplies. Dipper then chases after him, and Mabel enters to begin her plan to bust Mermando out.
Mabel next tells Mermando that her plan is to transport him using a cooler on the back of the Mystery Cart. Dipper quickly catches sight of Mabel and is further upset by the people he knows breaking in, and, as soon as Soos scales the fence, he tells him to go home. As Dipper grows suspicious of his sister, she distracts him by lying that Wendy is nearby in a bikini. She speeds out, closely followed by her brother in the Pool Mobile. An unnamed kid stuck in solitary confinement in pool jail then cheers for Mabel. Downtown, Dipper chases after his twin and assaults her with water balloons, and she continually defies him as she nears the lake. Stan, still at the pool, comes out of the bathroom after changing into his bathing suit and sits down on his desired chair.
While the twins speed through the forest, they pass Sheriff Blubs and Deputy Durland, who had been asleep until that moment. When Blubs says that he thinks someone may have sped by, Durland disregards it and tells Blubs it was probably a dream, and Blubs tells him that he finds every day to be a dream with his partner. At the edge of the lake, Dipper finally catches Mabel and instructs her to give back the cooler, but she says she cannot if she wishes to save Mermando, whom she reveals to be a merman. She then proceeds to take her friend out of the cooler and introduces him to her brother. Mermando greets Dipper, then expresses concern that he is dying from lack of water, and Mabel tells Dipper to give him "reverse CPR" to save him, and he regretfully complies. Mabel takes a picture of her brother for blackmail purposes, and Mermando thanks Dipper and asks why he did not simply roll him into the very close lake. Dipper then facepalms.
After the twins drop the merman off the dock into the lake, he asks how he will get his family to hear his calls, Mabel gives him a megaphone to aid him. Dipper tells her not to, however, as it is property of the pool and he will get fired if he loses track of it, and Mabel replies by telling him that she would do anything to help Mermando, even though she knows their romantic relationship will most likely not work out, and Dipper relates with his crush on Wendy. Mermando thanks Mabel and says he has never met anyone such as she, and Mabel says she feels the same way, except for a handful of other creatures. Mabel says goodbye, and Mermando kisses her before departing, much to Dipper's disgust. She is ecstatic about her first kiss, and teases Dipper by telling him that he had his in a way, as well. Before both parties depart, Mabel makes a reference to the ending of the movie Free Willy.
Back at the pool, Mr. Poolcheck penalizes Dipper for the damage to the pool property and fires him, telling him to hand in his whistle. As he eats it, Dipper slowly backs away. He proceeds to express his extreme anger and finally loses it after Soos throws several inner-tubes over the fence, chasing him away through the fence, and Tyler cheers him on. Though he is sad about losing the job, Wendy cheers him up when she reveals that she has been fired because she has been taking too many snacks, and the two go off to disobey rules elsewhere. Mabel, on the other hand, is still sad about losing Mermando but becomes happy once more when she receives several messages in bottles from him that describe how things are going. And, Grunkle Stan shocks Gideon when the boy finds Stan in his favorite pool chair, but reveals to be faking, as he expected Stan to try to take it and covered the whole chair in glue, and Stan calls for his niece and nephew to get their spatulas. In the end credits, the boy in solitary confinement is shown stuck there throughout the year.
- Written by:
- Directed by:
- Storyboards by:
- With the Voice Talents of:
- Kristen Schaal as Mabel Pines
- Jason Ritter as Dipper Pines
- Alex Hirsch as Stan Pines and Soos Ramirez
- Dee Bradley Baker as Waddles
- Matt Chapman as Mermando
- Linda Cardellini as Wendy Corduroy
- Keith Ferguson as Deputy Durland
- Will Forte as Tyler Cutebiker
- Michael Rianda as Mr. Poolcheck
- Kevin Michael Richardson as Sheriff Blubs
- Gregg Turkington as Toby Determined
- Thurop Van Orman as Gideon Gleeful
- Additional Voices:
- Casting by:
- Mabel has allegedly met cute vampires during the summer and didn't tell Dipper about it.
- Dipper reveals that Mabel has never kissed a guy because she always finds a way to ''mess it up''. Mabel reveals that Dipper also never had a first kiss.
- Mabel shows Mermando a picture of her family kickboxing, revealing that she and her parents or someone else in her family once tried a kickbox session or are kickboxers.
- Merpeople have been mentioned in previous episodes, including:
- Wax Stan, created in "Headhunters," is shown melting in the beginning of the episode.
- Mabel mentions the gnomes and Dipper thinking they were a zombie from "Tourist Trapped."
- Viewership: This episode was watched by 4.5 million viewers on its premiere night. This is the highest rated episode of the series.
- This is the only Gravity Falls episode to not have Alex Hirsch in a writing credit.
- In the original script, Stan says "blind the child" rather than "burn the child" when reflecting light into Gideon's face in an attempt to get him to vacate the chair that Stan wanted. This was changed because "blind the child" was deemed too graphic.
- When Mabel is receiving messages in bottles from Mermando, one of the bottles is labeled "Lebam," her name backwards.
- This is the first time we see Deputy Durland without his hat.
- Mabel again displays her comical unfamiliarity with other languages. Just as she didn't recognize Gideon's French in "The Hand That Rocks the Mabel," Mabel assumes the Spanish spoken by Mermando is a funny fish language.
- The cryptogram in the end credits reads "22-9-22-1-14 12-15-19 16-1-20-15-19 4-5 12-1 16-9-19-3-9-14-1." Once decoded it translates to "VIVAN LOS PATOS DE LA PISCINA," which is Spanish for "LONG LIVE THE POOL DUCKS."
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