Throughout their lives, twelve-year-old Maddy and her ten-year-old brother CJ have spent every summer holiday in Loutraki, Greece, where they love to swim with the dolphin Indigo. But the siblings have a secret: Maddy and Indigo can talk to each other. This summer, NATO is holding a conference in Athens, and American submarines are patrolling the nearby waters to prevent attacks. Unfortunately, the sonar used by the submarines is having devastating effects on the local dolphins and whales. When the president of the United States and his daughter Lucy arrive in Greece, Maddy and CJ enlist Lucy's help in alerting him to the dangers of sonar.
Maddy's Dolphin contains examples of:
- Atlantis: Described by Indigo as an ancient city full of waterways where humans and dolphins could all talk to each other. It was destroyed by a volcano and the resulting tidal wave, but the ruins are still inhabited by a dolphin known as the Keeper who has been entrusted with the stone scrolls of Atlantis's knowledge.
- "Be Quiet!" Nudge: The siblings call their Aunt Imogen to ask what the noise hurting dolphins could be. CJ is about to say that Indigo told Maddy about it, but Maddy kicks him to stop him from giving away their secret.
- Bookworm: Maddy and CJ's mother, Ishbel, loves reading. When she's engrossed in a book or magazine, she becomes totally oblivious to the outside world, which is convenient when her kids want to do something they know she'd disapprove of.
- Friendly, Playful Dolphin: Indigo loves playing with Maddy and CJ.
- Green Aesop: Human use of sonar kills sea animals that rely on it to navigate.
- Heroic Dolphin: Indigo and Maddy met when Indigo rescued her after she fell off a boat when she was two. Indigo and his dolphin friends also play a crucial role in rescuing Lucy after she's kidnapped by terrorists.
- Parental Substitute: Lucy's father is too busy to spend much time with her, so she's mostly raised by her bodyguard James. She even tells Maddy and CJ's parents that James is her father.
- Sapient Cetaceans: Whales and dolphins are all sentient and can all talk to each other.
- Sensory Overload: Sonar, especially low-frequency sonar, overwhelms the senses of dolphins and whales so that they don't know where they are. In their confusion, some of them beach themselves and die.
- Speaks Fluent Animal: Only a small number of humans and dolphins, such as Maddy and Indigo, can talk to each other. This means that if a cetacean other than Indigo wants to say something to CJ, first they have to say it to Indigo, and then Indigo has to translate to Maddy, and finally Maddy has to translate to CJ.
- They Would Cut You Up: CJ is the only other human who knows Maddy can talk to Indigo. They're afraid that if their secret got out, Maddy and Indigo would be experimented on.
