Ever since Monster (who sleeps under the bed) and Boy (who sleeps in the bed) met and became best friends, they've learned a lot about each other. So when Monster notices that Boy seems grumpy (Boy's mom has told him he's getting another baby sister), Monster decides to cheer his friend up. Together, they set out on a surprise adventure! But surprise: Boy's sister has tagged along. Together, they learn about how monster babies are born and get their names, before Boy and his sister find yet another surprise waiting for them back home.-- Publisher's description.