It's 1948, and twelve-year-old Nick is ready for the best summer ever. He's going to hang out with his best pal, Ace, and maybe with Penny too--she is a girl, but she has a great throwing arm! Then things get wild when a polar bear escapes from Milwaukee's city zoo and appears right on his block. They're all going to have to keep their eyes open now.

But Nick's grand plans start to crumble when Ace gets a paper route and Penny decides to share it with him. Now they're never around. Nick himself is working at his Uncle Spiro's frozen custard shop, but at least he gets free all-you-can-eat frozen custard. When Uncle Spiro opens a custard stand at the zoo, Nick volunteers to help--if that polar bear escapes again, he'll have a front row seat!

Only their competitor, Happy Harold, opens a stand of his own. Now Nick is scrambling to keep their customers, especially because Happy keeps playing dirty tricks. When Penny discovers that someone may have let the polar bear out on purpose, Nick suspects it might be Happy. With mysteries to solve and a whole zoo-full of monkey business, it looks like Nick's summer won't be so boring after all!