At the beach, the Sunnydale High swim team is celebrating their latest victory, while Xander complains about having such a big celebration just for the swim team. Buffy sulks on a rock, watching the ocean. Cameron Walker, another member of the swim team, approaches Buffy and starts sputtering out poetic words. Buffy tries to scare him off, but he says he’s looking to talk. Meanwhile, Jonathan Levinson is being dunked in a bucket of ice water by members of the team. Buffy rescues him and scares off the two bullying swimmers. However, Jonathan is angry with Buffy for saving him and tells her to leave him alone. The bullies, Dodd McAlvy and Gage Petronzi, walk off and talk about how weird Buffy is, and how she scares them. Dodd suddenly stops, while Gage keeps walking. He smells something bad, but goes on anyway. He walks by without seeing Dodd’s body, seemingly skinned apart. A mysterious creature walks into the sewer.
The next day, Willow is teaching a class, and as she is walking around checking people’s work on their computers, she sees Gage playing solitaire on his computer. She tells him off, but he doesn’t seem to care. After class, Principal Snyder tells Willow that they’re still unable to find a replacement for Ms. Calendar, so she’ll have to continue substituting for the time being. Willow informs Snyder that Gage is failing because he doesn’t show up for tests or do homework. Snyder is not interested; he tells her that, as Gage a member of the swim team, he’s under much more pressure than the rest of the students, and demands that she change Gage’s grade from an F to a D. Buffy comes to school with Cameron, and he tries to grope 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. Cameron tells Coach Marin, the coach of the swim team, and Snyder that Buffy was leading him on. The Coach, after telling Nurse Greenliegh to “take care of my boy”, turns to Buffy and tells her she shouldn’t dress so provocatively.
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 hears some weird noises, but it’s just Coach Marin telling him to hit the showers. While everyone studies in the library, Xander goes out for a drink. Heading for the vending machine, he bumps into Cameron. Xander teases him for getting beaten by Buffy after he tried to paw her. 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. Xander turns around and finds himself face-to-face with the creature responsible: a Gill Monster.
In the library, Cordelia sketches 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 team; Gage being the third best. Buffy is assigned to shadowing Gage — suspecting that he will be attacked next — while Willow interrogates Jonathan under a lamp. According to Jonathan, he indeed wanted revenge, but achieved it in a very different way: by peeing in the pool before practice one morning. Xander, who has been teased for his cowardice by Cordelia, overhears Marin and Snyder chatting. They need another swimmer, or they won’t be eligible for competition; they don’t even care if the guy can swim a stroke, so long as he shows up for practice. The next day, Gage grows suspicious of Buffy when she follows him around. At the Bronze that night, Buffy is confronted by Gage; she attempts to cover it up, but fails. She tells him that his life is in danger, but he doesn’t believe her and calls her a “psycho bitch” and a “head tripper”. He leaves, only to encounter Angelus in the parking lot. They complain briefly about Buffy, then Angelus turns into his “vamp face” and bites him, intending to “turn” Gage and recruit him for his war against Buffy. However, seconds later, Angelus stops and spits the blood out. Buffy comes to Gage’s rescue. A now-fearful Gage lets Buffy walk him home, while Buffy wonders why Angel spit the blood out.
The following day, Buffy sits in on swim practice along with Cordelia and Willow. The three girls wonder what could be killing off the swimmers, and Buffy suspects Gage had steroids in his blood. Cordelia comments on a cute new addition to the team, soon realizing that 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. At the beginning of swim practice, Gage stops in mid-lap to wave and smile at Buffy. Cordelia is delighted that she’s dating a member of the Sunnydale swim team, while Buffy and Willow tease him for being almost naked. Later, the team soaks in the sauna. Xander continues to babble, but no one pays attention. Claws come up through a grate, out of the sight of the team.
While Buffy waits for Gage to exit the boys’ locker room, she hears him yell. She charges in and finds him with a Gill Monster. Gage suddenly cries out in pain and falls to the ground: he rips open his shirt, and Buffy watches in horror as he becomes a Gill Monster himself. As Buffy fights the two monsters off, one grabs her from behind and bites her in the arm. Suddenly, Coach Marin saves Buffy just by bursting into the room; although Marin is alone and unarmed, the Gill Monsters flee. Buffy is sent to Nurse Greenliegh’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, asking where and how he can get some steroids. His fellow swimmers tell him that he’s already getting some, right here and now; it turns out the steroids are in the steam from the sauna.
Nurse Greenliegh tries to end Marin’s experiments, as he’s been attempting to improve his team’s performance by supplying them with steroids via the sauna-steam. To keep her from exposing him, he forces her through a grate into the sewers; there, the Gill Monsters devour her. Xander warns everyone that he’s had three exposures to the steam, and is in danger of becoming a Gill Monster himself. Buffy has Giles lock up the rest of the swim team, while she goes to talk with Coach Marin. She confronts him and demands information about the steroids he’s been giving them, telling him he’ll go to jail. Instead, the coach pulls a gun on Buffy and forces her into the same hole where Nurse Greenliegh was killed. The coach explains to Buffy that, while his monsters have already had their dinner, she’s there because “boys have other needs.”
Xander enters the locker room to check and see if he’s becoming “scaly.” A new Gill Monster emerges from the locker room and jumps in the pool. Cordelia, thinking it’s Xander, laments about his fate. Xander suddenly reappears beside her, right before “Sean” attacks Cordelia. 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, who is twice his size, and knocks the coach out. Xander looks down into the hole and sees Buffy, up to her waist in sewer water, surrounded by Gill Monsters who are about to attack and slaughter her. He pulls her out before they can do so. (Note this is the second time in one and the same episode that someone has conveniently shown up at the last possible moment to rescue Buffy, of all people, from certain doom.) Coach Marin wakes up and bashes Xander’s head with a pipe wrench, incapacitating but not killing him. Marin tries to do the same with Buffy, but Buffy ducks and trips him — accidentally causing him to fall into the hole. She tries to catch him, but misses. Being no match for the Gill Monsters in their element, Buffy can only observe as they close in and rip the Coach to pieces — literally biting the hand that feeds them. Xander recovers just in time to join her in witnessing Marin’s demise.
The next day, everyone recaps; Xander and the surviving swim team members are given treatments created by the hospital’s toxicology lab that will stop the steroids. Meanwhile, the Gill Monsters swim out into the ocean.
This is the first of three episodes where we see a beach, the others being "Anne" and "Buffy vs. Dracula".
This is the first of two episodes where Willow interrogates Jonathan, the other being "Earshot".
Willow's line to Jonathan "You wanted revenge, didn't you? So you delved into the black arts and conjured up a hellbeast from the ocean's depths to wreak your vengeance" is ironic, and also foreshadows both of their futures. Dark Willow will seek vengeance on Jonathan for Tara's death and Jonathan, as a member of the Trio, will conjure demons.
Jonathan, yet again being bullied, in this episode is setting up for his suicide attempt in "Earshot".
The nurse and coach are the fifth and sixth members of the Sunnydale High faculty to die in the course of the series.
This episode features one of the most notable examples of role reversal in the series' horror genre: Gage (a strapping and muscular male athlete) asks Buffy (a petite, sexy blonde girl) to walk him home because he's afraid.
Xander's line "In other words, 'this was no boating accident'" is a reference to Jaws. "This is not a boat accident" was uttered by Richard Dreyfuss' character after he examined a dead body on the beach.
Cordelia attempted to refer to the movie The Creature from the Black Lagoon when she was afraid that Xander would turn in to a Gill Monster, but she accidentally said "The Creature from the Blue Lagoon." Xander corrected her by reminding her that The Blue Lagoon was a movie starring Brooke Shields. The scene where the mutated swim team force themselves on Buffy is also reminiscent of infamous B-movie 'Humanoids from the Deep'.
When Xander comes out of the locker room after helping Buffy keep an eye on Gage, he tells her that Gage will be out in a couple of minutes after he puts his sneakers on. Gage is then shown putting on his shoes, but they aren't sneakers.
At the beginning Willow says that the solitaire cards Gage is looking at have naked women on them, but they aren't actually naked.
While in the lounge room watching Gage, the magazine Buffy is hiding behind changes between shots.
Xander - "That is wrong. Big, fat, spanking wrong. It's a slap in the face to every one of us that studied hard and worked long hours to earn our D's."
Carl Marin - "Take good care of my boy, Ruthie."
Nurse Greenleigh - "I always do."
Carl Marin - (turns to Buffy) "And you... Try to dress more appropriately from now on. This isn't a dance club."
Cordelia - "It’s about time our school excelled at something."
Willow - "Hmm. You’re forgetting our high mortality rate."
Xander - "We’re number one!"
Xander - "I’m undercover."
Buffy - "Not under much."
Cordelia - "I’m dating a member of the Sunnydale Swim Team."
Buffy - "Now you can die happy."
Buffy - (to Giles) "And raise the possibility that someone brought forth this sea monster from whence it came to exact that revenge. ‘From whence it came’? I’m spending way too much time around you."
Willow - "So, you delved into the black arts and conjured up a hell-beast from the ocean's depths to reek your vengeance. Didn’t you?"
Jonathan - "What? No. I snuck in yesterday and peed in the pool."
Willow - "Oh. Ew!"
Buffy - "I think we should lock up the rest of the swim team before they get in touch with their inner halibut."
Cordelia - (hears a door open, and assumes it’s Xander) "Any gills yet?" (Behind her, a creature comes in and jumps into the pool; she looks into the pool, but at first can’t make out the monster because of the splash on the surface) "Xander, what are you doing?" (giggles) "Xander?" (The creature swims beyond the splash, and Cordelia can see it clearly now) "Oh, my God! Xander!" (She walks along the edge of the pool, following the monster as it swims under water; she now sounds very upset) "It’s me, Cordelia? I know you can’t answer me, but... God, this is all my fault. You joined the swim team to impress me. You were so courageous. And you looked really hot in those Speedo’s." (chuckles) "And I want you to know that I still care about you, no matter what you look like. And... and we can still date. Or, or not. I mean... I understand if you wanna see other fish." (crouches by the edge) "I’ll do everything I can to make your quality of life better... whether that means little bath toys or whatever." (Xander walks up behind her)
Xander - "Uh, Cordy?" (Cordelia spins around quickly, startled, and draws a frightened breath; Xander points into the pool) "That’s not me." (They both look at the creature, which suddenly makes a lunge for them from the water; Cordelia screams, and Xander pulls her up and away before they run from the pool area)
Cordelia - "Oh, my God!"
Cordelia - "I think we can safely say we’ve found Sean. He was in the pool skinless-dipping."
Carl Marin - (leads Buffy to the same hole through which he shoved Nurse Greenleigh) "You got some imagination, missy."
Buffy - "Oh, well right now, I’m imagining you in jail. You’re wearing a big, orange suit, and, oh, look, the guards are beating you up."
Buffy - (stops following, in a stern tone) "Tell me what’s in the steam."
Carl Marin - (faces her) "After the fall of the Soviet Union, documents came into light detailing experiments with fish DNA on their Olympic swimmers. Tarpon... mako, shark... But they couldn’t crack it."
Buffy - "And you did... sort of. Why?"
Carl Marin - "What kind of question is that? For the win! To make my team the best they could be! Do you understand we have a shot at the state championship?"
Buffy - "Do you understand that I don’t care? It’s over. There’s not gonna be any swim team."
Carl Marin - "Boy, when they were handin’ out school spirit, you didn’t even stand in line, did you?"
Buffy - "No. I was in the line for shred of sanity." (The coach reaches into the desk behind him, pulls out a Beretta 9mm semi-automatic pistol, pulls back the hammer and points it at her) "Which you obviously skipped."
Carl Marin - "Get in the hole!" (He indicates with the gun; Buffy looks down at the hole and back up at the coach) "In! Now!" (She sits down on the edge with her legs dangling in and looks up at him)
Buffy - "This isn’t over."
Carl Marin - "In!" (She gives him one last look and drops herself into the water below. It’s about a fifteen-foot drop, so she submerges completely and comes back up, soaked through and wiping the water from her eyes, and looks up at the coach leaning over the hole) "You think I don’t care about my boys. But I do. They count on me." (Buffy understands now, and looks around for the monsters; when she turns back around, she screams at the sight of what's left of Nurse Greenleigh’s body as it floats by before she looks back up at the coach)
Buffy - "So, what, you’re just gonna feed me to ’em?"
Carl Marin - "Oh, they’ve already had their dinner. But boys have other needs."
Xander - (walks into the pool equipment room, where he spots the coach leaning over the hole and gloating) "What’s up, coach?"
Carl Marin - (looks back, startled) "Oh! Harris. Uh..." (stands up) "how you feelin’?"
Xander - "Little dry. Nothing a lemon butter sauce won’t cure." (They both chuckle nervously.) "Where’s Buffy?" (The coach's eyes focus on his gun laying on a barrel just behind Xander. Cut to below; Buffy keeps looking for the sea monsters. One suddenly rises up behind her and lunges at her. She quickly grabs it and throws it aside. Another one comes for her, and she deflects it into the wall, but loses her balance and falls underwater. One grabs her leg, and she kicks out. She grabs it and throws it off of her. Standing again, she grabs another one's arm as it attacks her and flips it over into the wall. Cut above; Carl Marin makes a grab for his gun, but Xander grabs his arm, pounds it into the barrel and makes him drop it. He elbows the coach in the face, who falls over unconscious.)
Xander - "Hold on!" (He pulls again, but he's not as strong as he thinks and it goes slowly. One of the monsters leaps up and grabs Buffy's leg again; she kicks it with her free leg and it goes tumbling down. Xander pulls again as Buffy raises her legs up out of the creature's reach. Slowly she emerges from the hole and climbs onto the floor, coughing and panting)
Buffy - "Thanks."
Xander - "Just doin' my part for our team." (Behind him the coach has regained consciousness, and hits Xander across the back with a large pipe wrench)
Buffy - "Oh!" (He starts to take a swing at Buffy with the pipe wrench, but she ducks away and trips him with her leg, causing him to fall and scream while rolling into the hole; Buffy reacts fast and grabs his ankle)
Carl Marin - "Don't let go! Don't let go of me! Hold me!" (Buffy can't maintain her grip, and the coach falls into the water below; he stands up in the water, shaking it out of his face and coughing)
Buffy - (reaches down) "Grab my hand!" (He's too busy being worried about his sea monsters surrounding him to listen to what Buffy is saying)
Carl Marin - "Boys! Boys, uh, now, now, boys! No! I..." (They attack him) "No, boys!" (He screams and struggles as they pull him under; Buffy pulls her arm back up)