In a whimsical rhyme, we learn that Calvin has been forced to babysit Mrs. O'Leary's zoo animals, a monumental task that goes awry when all the animals leave the house to take a walk. Once outside, singing and enjoying the summer sun, the animals bump into Mortimer the mouse—and the chase is on. Bedlam, of course, ensues as the animals blaze a path of destruction through the city chasing the mouse while Calvin runs behind them yelling at the rampaging herd to stop.