At the beach, the Sunnydale High Swim Team is celebrating their latest victory. While many people party, and while Xander complains about the swim team having a celebration, Buffy sulks on a rock watching the ocean. A guy named Cameron approaches Buffy and starts sputtering out poetic words. Buffy tries to scare him off, but he says he's looking to talk, nothing more. All of a sudden, Jonathan is dunked in a bucket of ice water. Buffy rescues him and scares off the two bullies.
The guys walk off and talk about how weird Buffy is. Then Dodd stops, while Gage keeps walking. Gage smells something weird, but keeps going. He walks by without seeing Dodd's skin, which is on the ground with nothing left inside. We see a creature walk through the sewer. The next day, Willow is teaching a class, and as she is going round seeing people work, she sees Gage playing card games with naked women on the cards. She tells him off, but he doesn't seem to care. Principal Snyder confronts her about Gage after class, having just told her that they can't get a new teacher for a while and she will have to continue covering (which pleases her). He starts talking about Gage, and Willow tells him that he doesn't turn up for tests or do homework, or indeed anything at all. He is not interested, and tells her that as Gage is on the swim team, he shouldn't be expected to do work and that she should change his grade. Buffy comes to school with Cameron, and he makes a pass at her. She hits his head against the steering wheel and breaks his nose. Snyder happens to see this; he and Cameron blame Buffy for the altercation.
Later, Buffy rants to Giles and her friends about the incident with Cameron. Giles informs her that they found Dodd's remains on the beach. Cameron hits the sauna and, while he's in there, hears some weird noises. The boys' Swim Team Coach Carl Marin bursts in and announces that Cameron has had enough. While everyone studies in the library, Xander goes out for a drink. Heading for the vending machine, he bumps into Cameron. Xander laughs at him for trying to hit on Buffy. Cameron heads off to the cafeteria for a snack, while Xander tries to decide between grape and orange soda. Cameron screams, and Xander rushes into the cafeteria. He finds the place in shambles, with Cameron's skin on the ground. As Xander turns around, the creature responsible for the mess - a Gill Monster - appears behind him.
In the library, Cordelia tries to sketch the creature that Xander saw before running off "like a woman." Buffy and Willow return, having been informed that Cameron and Dodd were the two best swimmers on the boys' team; after them was Gage. Buffy is assigned to shadowing Gage - suspecting that he will be next in line, as it were - while Willow interrogates Jonathan under a lamp. According to Jonathan, he indeed wanted revenge, but stopped after peeing in the pool; he wasn't responsible for the guys' deaths. The next day, Gage grows suspicious of Buffy when she follows him around. At the Bronze that night, Buffy is caught in the act by Gage; she attempts to cover it up, but fails. He leaves, only to encounter Angelus in the parking lot. They talk briefly about Buffy, then Angelus "vamps out" and bites him. Seconds later, he stops and spits the blood out. Buffy comes to Gage's rescue and walks him home. She suspects that Angelus spat Gage's blood out because there were steroids in it.
The following day, Buffy, Willow and Cordelia sit in on swim practice. The three girls chat, still wondering what could be killing off the swimmers. Cordelia comments on a cute new addition to the team, and then realizes it's Xander. He walks over and informs them that he just made the team. Willow tells Buffy that Jonathan wasn't responsible for the swimmers' deaths, but instead got his revenge by peeing in the pool. Later, the team soaks in the sauna. There, Xander continues to babble, but no one pays attention. Claws come up through a grate; still no one pays attention.
While Buffy waits for Gage to exit the boys' locker room, she hears him yell. She charges in and finds Gage with a giant fish creature: a Gill Monster! Gage then cries out in pain and falls to the ground. He rips open his shirt, followed by his skin, and becomes a Gill Monster himself. Buffy tries to fight them off, but one grabs her from behind and bites her arm. Coach Marin comes in and saves Buffy, who is sent to Nurse Greenleigh's office. Marin is told that members of his team are becoming Gill Monsters. Buffy and Giles go hunting for Gill Monsters in the sewers, but fail to find any. Meanwhile, Xander again hits the sauna; there, he tries to find out where and how he can get some steroids. His fellow swimmers tell him that he's already getting some right now; the steroids are in the steam from the sauna.
Nurse Greenleigh tries to end the experiments of Coach Marin, who has been attempting to improve his swim team's performance by supplying them with steroids via the sauna-steam. He throws her to the Gill Monsters, who promptly devour her. Xander warns everyone that he's had three exposures to the steam; he is understandably worried about becoming a Gill Monster himself. Buffy tells Giles to lock up the rest of the swim team, while she goes to talk with Coach Marin. She discovers too much about his connection with what's happening. Pulling a gun on her, he forces her into the same grotto where Nurse Greenleigh met her demise. Marin tells Buffy that the Gill Monsters will kill her, but not by eating her: "Boys have other needs."
Xander and Cordelia help round up the boys' swim team members. Xander enters the locker room to check and see if he's becoming "scaly." A Gill Monster who used to be Jason emerges from the locker room and jumps in the pool. Cordy thinks it's Xander and carries on about how much she cares for him. Suddenly, Xander reappears beside her, right before "Shawn" attacks Cordy, just missing her. They flee back to the library. Xander decides that, since Buffy hasn't returned, he should go after her. He meets up with Coach Marin and fights him for the gun. Xander disarms Marin and knocks the coach out. Xander looks down into the grotto and sees Buffy, up to her waist in fetid water, being surrounded and attacked by Gill Monsters who are about to finish her off. He pulls her out with no time to spare. Then Coach Marin bashes Xander's head with a pipe wrench from behind, incapacitating him. Marin tries to do the same to Buffy, but misses; his swing carries him off balance and he falls into the grotto. The Gill Monsters close in and rip him to pieces.
The next day, everyone recaps. Meanwhile, the Gill Monsters swim out into the ocean.