Gracie, a high school-aged girl, lives in a small New York town called Little Spindle, near the city of Greater Spindle. The town and city are regular summer vacation destinations; however, many believe the lake in Little Spindle to be haunted by a monster resembling the Loch Ness monster. An older woman in the town named Annalee Saperstein is19 surrounded by intrigue and stories. Gracie's favorite story about Annalee is that in the 1980s, when she lived in NYC, she took in a girl who was kicked out of her home after giving birth to a child. The girl claimed the father was a being from the Hudson Sea, and Annalee was the only one who believed her. The girl's daughter would only eat sea food and grew up to be an incredibly beautiful supermodel, which gave her the means to buy her mother an Upper West Side penthouse and give Annalee the money to move to Little Spindle, where she now owns the Dairy Queen. The townspeople come to the booth where she sits in the Dairy Queen for wisdom about their strange problems. One day, Gracie believes she sees the lake monster, nicknamed by the town "Idgy Pidgy," at the cove she frequents by the lake. She knows what she saw, but she also doubts herself. She goes to Annalee to ask her what she knows about the monster. Annalee tells her to ask a summer kid who comes into the Dairy Queen every day. Gracie knows him—Eli Cruddy—and thinks of him as a weird kid with no friends who spends all his time in the library. She describes him, mildly repulsed, as "moist." The two teenagers talk and start to become friends while trying to find out about the lake monster. However, summer soon ends and Eli returns to the city. The next summer, the two meet up again at the Dairy Queen and spend the summer together. Eli finds a dusty record room in the basement of the library and the two listen to music. They spend time together without talking about Idgy Pidgy once. Gracie hasn't mentioned him to her other friends—because she finds her friendship with him hard to explain and feels embarrassed—but one day her other friends, Mosey and Lila, run into them outside a restaurant and join them. Gracie realizes she also wants to keep her friendship with Eli to herself because she feels alone around her other friends, but not around Eli. A summer or two later, Gracie realizes Eli is no longer "moist" and that she's attracted to him. She spends the entire summer suppressing her attraction. She tries to reignite an old crush to make her forget about her crush on Eli but feels nothing for the boy. At the end of the summer, she says she wishes he had a phone so they can stay in touch. Gracie goes to junior prom with a boy she doesn't really like, gets drunk, kisses him, and throws up. When summer comes, she doesn't go to see Eli at the Diary Queen because she doesn't want to spend another summer pining after him, but he finds her on the beach and kisses her. They date all summer, but they don't go to each others' houses. At the end of the summer, Gracie says she's thinking about moving to NYC. She'd hoped that Eli would be happy, since she would live near him, but he isn't. She storms off, and she doesn't meet him to say goodbye on his last day in Little Spindle. A week later, she goes to see Annalee, who indicates she should try to say goodbye even though Eli is gone. Gracie goes to the cove and finds Eli slumped in the water, his skin nearly blue. He says he is dying because he overstayed the three months the lake allows him to be human. He is the lake creature she saw. She finds a cloak of scales in his backpack and puts it over both of them, offering herself to the lake in exchange for Eli's life. The two become lake creatures who spend three months a year as human. They go with Annalee and Gracie's mom to visit the girl from the story (who had a child with the Hudson river spirit) in her penthouse on the Upper West Side.