When Fresno is hit with an unexpected outbreak of deadly scarlet fever, Andi Carter is scared. Her town friends are getting sick and school is closed until it passes. Far from the danger, at least Andi has her horse Taffy and plenty of work on the Circle C ranch to distract her.

But is she really safe? First one ranch hand, then two, and then her brother Mitch fall ill. Mother is convinced that the only way to protect Andi from this terrible disease is to send her far away to fussy Aunt Rebecca in San Francisco. The noisy city and Aunt Rebecca's bossy ways may be worse than scarlet fever. But then Andi has an idea that might help save her friends back home . . .