After eleven-year-old Ginika Orendu's family is evicted from their flat in London, her parents send her to spend the summer in her grandparents' boarding house in the Cumbrian seaside town of Bridleways Bay. At first she desperately misses her home, her parents, and her friends. Then she meets Peri, a not-quite-human boy who lives in the ocean and wears clothes made of seaweed. As Ginika gets to know Peri, she learns more about his species, the sea people, while he learns about humanity.
The Storm Swimmer contains examples of:
- "Be Quiet!" Nudge: At the beginning of the book, Ginika argues with her parents about why she should be allowed to stay with them in London in their campervan instead of being sent away. Her dad says, 'We need to get you far, far away, where you'll be safe.' Her mum nudges him with her little finger. Ginika realises that her parents think they're in danger and don't want Ginika to know about it.
- Dream Reality Check: When Ginika first sees the sea people, she pinches and slaps herself to make sure she's not dreaming.
- The Friend Nobody Likes: Scarlett, a girl whose parents own the caravan park, is disliked by the other kids for being bossy and rude. Some kids put up with her anyway because her dad owns a pump track and a water park. Scarlett is aware of how most people feel about her and is upset about it, but doesn't know how to make people like her.
- Happy Dance: After Ginika sees Peri for the first time, she dances all the way back to Cormorant Heights.
- Hide Your Otherness: Ginika takes Peri on a tour of the town in a bicycle trailer. Peri keeps his legs, which are shaped differently from human legs, hidden inside the trailer, and wears a floppy hat to hide his hair, which takes on the colours of his surroundings. He carries a bottle full of seawater to keep his eyes moist. There's a local charity that takes sick and disabled kids on seaside holidays, and Ginika pretends Peri is one of the kids from the program to explain why he doesn't walk or talk.
- Human Subspecies: Ginika's grandpa thinks the sea people are descended from a population of humans who were stranded on an island that very gradually sank into the ocean, giving the inhabitants time to adapt to the sea.
- Intrigued by Humanity: Peri is fascinated by humans and especially by artificial lights. Unfortunately, the magnetic fields generated by human technology affect his navigational abilities, and too much exposure to human-generated electricity causes him to fall ill.
- Language Barrier: Ginika and Peri learn a few words of each other's languages, but mostly communicate through body language and mime. Late in the book, Ginika takes Peri into the laundry room, where her grandpa keeps his National Geographic collection, and they both use pictures from the magazines to teach each other about their lives.
- Loan Shark: Ginika's parents eventually tell her the real reason they sent her away instead of letting her stay with them in the campervan: they owed money to loan sharks and were afraid they'd go after Ginika if they couldn't pay up.
- No Animals Allowed: Pets aren't allowed in Cormorant Heights, but one lodger has a chihuahua he smuggles in and out in a bag.
- One-Steve Limit: Averted. The local caravan park has three girls named Olivia who are all friends with Scarlett. Ginika thinks of them as the smiley Olivia, the starey Olivia, and the fidgety Olivia. Nearly everyone calls them 'the Olivias', even though the starey Olivia protests that they're separate people.
- Photographic Memory: The sea people instantly form a detailed 3D mental map of every place they visit. The same ability allows Peri to memorise the pictures on each page of National Geographic as soon as he sees them.
- The Runaway: When Peri's family passes by Bridleways Bay, Peri is so fascinated by the sight of a human town that after the family moves on, Peri runs away and returns to the bay.
- Shown Their Work: An author's note describes many aspects of sea people physiology that didn't make it into the novel, showing the research the author did on marine life and her efforts to make the sea people biologically plausible.
- The Unpronounceable: The sea people's mouths are shaped differently than human ones, allowing them to make clicks and trills that are impossible for humans. Peri's real name is pronounced 'Eee-click-peri.' Most English words are also unpronounceable to Peri, including Ginika's name. He calls her 'Gnka,' trilling the G.
