Mandy Hope loves animals and knows lots about them too - both her parents are vets! So Mandy's always able to help her friends with their pet problems... Mandy and James' school-friend Martin has been given a pair of fancy rats for his birthday. Cheddar and Pickle love to race around their 'Incredible Rat Run'. At first, Mandy finds that Pickle is the fastest. But then Pickle's times begin to slow down. Then Pickle gets stuck in the run. What on earth is wrong with the little rat? Then Mandy's father investigates. It turns out that Pickle is a she, not a he, and she's pregnant! Now Martin has to keep Cheddar and Pickle apart - but Mandy knows Pickle will be back on prize-winning form as soon as she's had her babies...