The PastLily's sister Vivian is born with a heart defect, and Lily is charged by their parents to care for her. Vivian hero-worships Lily as she grows, imitating everything her big sister does... which leads them to become synchronized swimmers. Lily never forgets that she must always put her sister's happiness before her own.
The PresentLily and Vivian find themselves alone on Chuck's half-birthday, one and a half years after her death. Vivian spots an ad in the paper for a synchronized swimming show called the Aquacade—the very same show that they liked to take Chuck to, to celebrate her half-birthdays—and decide to honor Chuck's memory by attending.Chuck is also in attendance that evening: Ned has treated her, Emerson, Olive, and himself to tickets for her half-birthday present. That night's headliners are the Aquadolls, a pair of synchronized swimming sisters (Blanche and Coral Ramora) who used to be the Darling Mermaid Darlings' chief rivals. The pairs of sisters exchange barbs before the show.During the opening number, Blanche is eaten by the show's shark.The next day, Emerson is called upon to prove that the shark was released by someone other than its owner, and the rest of the gang come along to investigate. He is handed the perfect cover to investigate the Aquacade's operations from the inside when the Aquacade's owner persuades Lily and Vivian to sign on: he tags along as their coach; Olive, as their hairstylist and wardrobe manager; and Ned, as their business manager.The Aquacade's owner is so pleased that he begins to plan a European tour, to which the aunts—to everyone's surprise—assent. Ned relays this to Chuck, hoping she'll be pleased that her reclusive aunts are shaking off the mantle of grief and resuming their lives... but Chuck is heartbroken and furious. Staying away from her aunts has been tolerable only because she is able to stay in their lives, however peripherally.Ned and Chuck separately try to sabotage the Darling Mermaid Darlings' first performance, so as to call off the European tour—but they are caught at it by Emerson, who is not pleased at how low his usually upstanding associates have sunk.During the performance, the murderer attempts another pair of murders, but is stopped by Emerson, Olive, and Ned.The next morning, Ned and Chuck talk. Ned confesses that keeping her from seeing her aunts was always more about keeping himself safe than about keeping her safe, even if he didn't realize it or didn't want to admit it to himself, and ends with the decision to let her reveal herself to her aunts.Lily has awoken that morning proud of herself and happy with her life for the first time in years—but her happiness is shattered when she sees Coral leave the house and Vivian in tears. Coral has told Vivian Lily's secret—that Lily was the one who had borne Charles Charles' daughter Charlotte while he was engaged to Vivian. Vivian, heartbroken and coldly furious, orders Lily out of her life... but as she flings open the door to escort Lily out, the sisters find themselves face-to-face with Charlotte herself, bearing a bouquet of daisies and saying, "I'm alive..."
The FutureThe Darling Mermaid Darlings, far from retiring, go on to tour the world twice. Emerson's daughter Penny really does find him through his book. And Olive and Randy Mann marry and open a macaroni and cheese restaurant called The Intrepid Cow.