Mairi's brother Robbie says swimming is easy, you just pretend you're a mermaid. But that doesn't help Mairi, she's been trying all summer. Besides, she isn't even sure that mermaids are real. That is, until she finds a tiny mermaid, and rescues her from a ferocious crab. The mermaid must get back to her mother. Mairi agrees to carry her out past the rocks to where it's safe. Before she knows it, Mairi is swept off her feet by a wave and finds herself swimming! Together they reach the rock where the mermaids live. Grateful to Mairi, the mermaid's mother asks how she can repay her kindness. Mairi tells the mermaids that she wants to swim like one of them. Soon she has mastered breaststroke and crawl, backstroke and butterfly. She even visits their underwater grotto.
