Freddy the mouse’s cousin Isobel is allergic to milk. Her allergy is severe enough that she can’t even get milk on her skin without having a reaction. On a trip to the seaside, Isobel’s mother warns her friends about the allergy, but when they meet Swish the Mermaid who is eating an icecream, Isobel can’t resist and tries a little. Isobel starts to feel unwell, and The ‘Mermaid Doctor’ is called. He injects special medicine into Isobel’s leg and she feels better. The doctor reminds her that she must always carry medicine with her.