The story follows Koichi Makino and his younger childhood friend, Shizuku Sonoshiro. Despite their mutual feelings for one another, their relationship remains at a standstill due to Koichi’s aloofness and inability to take the first step. In an attempt to get his attention, Shizuku joins the school’s swim team, even though she isn't athletic. The turning point occurs when Koichi makes a thoughtless remark that deeply hurts Shizuku. While crying alone in a park, she is approached by the swim team's temporary coach, Yuji Sawamura. Appearing kind and comforting at first, Sawamura hides a dark, criminal nature. He lures her to his home, where he takes advantage of her emotional vulnerability. This encounter marks the beginning of a downward spiral for Shizuku. Under Sawamura's influence, she begins living a double life. Through constant manipulation and "secret lessons" after practice, Shizuku’s original devotion to Koichi is slowly eroded and replaced by a dependency on the pleasure and corruption introduced by Sawamura.