A mermaid lives in the waters near a Scottish fishing village. Everyone believes in and fears her, except Eric Anderson, who laughs at everyone else's belief in her. But when the mermaid punishes him for his mockery by almost drowning him and his crew, he realises that not only does she exist, but he can never again safely fish near the village. Eric leaves his home and travels abroad, but he still sends back gifts for his grandchildren, Jon and Anna, including a beautiful jade comb. When Anna accidentally summons the mermaid again, the mermaid demands Anna's comb. Anna refuses, even when the mermaid threatens to punish the whole village.
The Mermaid Summer contains examples of:
- Magic Music: The mermaid's song can mind control anyone into doing anything she wants them to. As a threat, she uses it to force Anna to almost drown herself. In the end, she uses it to make Eric come back home.
- Message in a Bottle: Anna at first wants to write to Eric this way, in the hopes that the bottle will be found by someone who knows where Eric is at the moment. But Jimsie Jamieson, the oldest fisherman in the village, explains to her that the tide would bring it back to shore before it got very far. Instead, he tells her and Jon to write to the Seamen's Mission at the home port of Eric's ship so he can collect the letters every time the ship returns to Scotland.
- Sibling Rivalry: The mermaid's motivation for wanting to take Anna's comb is to have more beautiful things than her sisters so they'll be jealous of her.
- Sirens Are Mermaids: The mermaid's main use for her voice is forcing people who get on her bad side to crash their boats against a group of pinnacles called The Drongs.
- Three Wishes: Jon and Anna capture the mermaid and threaten to cut off her hair, the source of her power, if she doesn't grant them three wishes. Jon wishes that no one from their village will ever be drowned again at sea, and that no boat he builds will ever sink. Anna wishes for Eric to come home.
