The beach is the site for the victory party of the Sunnydale High swim team, and everyone's celebrating except Buffy. One of the team members, Cameron, approaches the lonely Slayer and keeps her company. Another team member, Dodd, mocks Jonathan in front of everybody, so Buffy steps in to stop the foolishness. Dodd and fellow member Gage then leave the party and walk along the shore. Suddenly, Dodd disappears without Gage even noticing. He smells a strong odor coming from somewhere, so he decides to return to the party. He doesn't hear Dodd quietly scream in agony, nor does he notice the scaly creature walking away from Dodd's remains.

The next morning, Principal Snyder stops by the computer classroom to talk to Willow about one of her students, Gage. He is currently getting an "F" in the class, and Snyder encourages her to work his grade into a passing one. Meanwhile, Cameron gives a Buffy a ride to school, then tries to put the moves on her. She retaliates by busting his nose on the steering wheel. Principal Snyder sees this and calls the two of them to Nurse Greenliegh's office. The swim team coach, Marin, meets them there, and they discuss their chances for winning the state title. In the library, Buffy learns that Dodd's insides were eaten from some mysterious creature, leaving only his skin behind. Hours later, Xander takes a study break and heads to the vending machine. He bumps into Cameron, who is on his way to the cafeteria. Cam notices a foul smell when he enters the cafeteria, and a horrific scream follows soon after. Xander follows the sound, but he's too late, for all that remains of Cameron is his skin on the cafeteria floor. Xander then turns around and encounters the scaly creature that most likely killed Cameron.

Later, Xander explains to Giles and Cordy that the creature got away before he could get a good look at it. Without Cameron gone, Giles determines the pattern of the killing is connected to the swim team ranks. Dodd and Cameron were the first and second best swimmers on the team, respectively. Since Gage is the third best, he would most likely be the creature's next target. Buffy decides to keep an eye on Gage, while Willow interrogates Jonathan, who has tried out for the team several times but never made it on. All Willow learns is that Jonathan peed in the pool. Later that night, Gage confronts Buffy about her spying on him. She tries to come up with a good excuse, then simply tells Gage what's really going on. He doesn't buy it, so he turns and walks out. Outside, he meets Angel, who promptly sinks his teeth into Gage's neck, hoping to turn him into a demon. Buffy arrives on the scene just in time to witness Angel suddenly withdrawing his fangs from Gage's neck and spitting the blood out. He tosses Gage's weakened body at Buffy and makes his escape, all the while spitting out the blood in disgust. Gage believes Buffy now, so he asks her to walk him home.

At swim practice the next day, Buffy tells Willow and Cordy about Angel's distaste for Gage's blood. She believes that Gage may have been taking steroids, which might have turned Angel off. Willow then considers the possibility that the steroids might be what's attracting the creature. They then find out that Xander has joined the swim team. The position was open due to the recent deaths, so Xander took it as an opportunity to spy on Gage when Buffy couldn't. After spending some time in the steam room, Xander gets dressed and meets Buffy outside of the locker room, telling her that Gage is still inside. She soon hears Gage screaming and runs inside, where she sees the creature standing in front of Gage. She gets between the two of them, but something's wrong with Gage. To her horror, Buffy watches as Gage tears away his own skin in agony before emerging as the same type of creature standing behind her. Gage's former skin drops to the floor, and he quickly attacks Buffy. The two of them are too much for Buffy, and when her guard drops, one of them bite into her arm. Coach Marin arrives to pull Buffy out of danger, and the two creatures escape through a hole in the floor. Buffy and Giles question Coach Marin, but he denies any knowledge of substance abuse in his team. They can't figure out what's going on, so Buffy assigns Xander to learn more about the team's drug use while she and Willow talk to Nurse Greenliegh. However, Nurse Greenliegh is already being led to her doom by Coach Marin, who won't listen to her pleas to stop the experiments. He drops her into the sewer below, where the fish creatures quickly feed on her.

In the steam room, Xander learns from one of the other members that the steroids are in the steam itself. This means that Xander has been exposed to the same substance that transformed Dodd, Cameron, and Gage. He tells the others that he's already been in the steam room three times, so Buffy talks to Coach Marin. Eventually, he tells her about Russian experiments with fish DNA on their Olympic swimmers. He then draws a gun and orders Buffy to drop into the sewer so his boys could satisfy their "other needs". After dropping down, Buffy sees the chewed-up remains of Nurse Greenliegh floating in the murky water. Upstairs by the pool, Cordy mistakes a newly-emerged fish creature for Xander. She expresses her sorrow for his fate, but Xander shows up again, just in time to save her from the creature's attack. They return to the library, where Giles has locked up the remaining members of the swim team. Xander then goes to see Coach Marin, and they engage in a struggle over the gun. Down below, Buffy fends off the creatures until they finally surround her from all angles. Xander knocks Coach Marin down and extends his arm through the hole for Buffy. She leaps up, grabs Xander's hand, and is pulled to safety. Coach Marin gets up for one last strike, but Buffy is ready this time. She sweeps him off his feet, sending Marin into the sewer with his swimmers. They converge on him, leaving Coach Marin with nothing to do but scream. Later, Xander tells the others that he has to get a blood transfusion along with the rest of the team, hoping that will prevent them from changing. As they speak, the fish creatures swim out into the ocean, against the current.