Harvey the muskrat is convinced that his big sister Mildred is loudmouthed, bossy, and completely rotten. Mildred is just as sure that Harvey is selfish, inconsiderate, and no good. Because they are constantly arguing, they refuse to play together and spend their days feeling lonely and bored. When Mildred gets invited to a fun party where little brothers are strictly forbidden, Harvey decides to create his own fun. He builds a super secret club in a hidden hideout where big sisters are not allowed to enter. But as the afternoon drags on, Harvey realizes that sitting alone in a secret hideout is not very exciting. Watch what happens when a stubborn brother and sister discover that playing together is much better than being alone.