Peter Hatcher is horrified to learn of his family's plans to spend their summer in a vacation home alongside the Tubmans, the family of his archrival Sheila Tubman, located in Southwest Harbor, Maine.
On the other hand, his younger brother Fudge anticipates the vacation because of his plans to marry Sheila as a means of protection against the supposed "monsters" hiding beneath his bed, knowing that spouses often share a bed. Fortunately, this wish is pacified and dropped after a newfound friend in a small girl named Mitzi provides him with a bottle containing her grandmother's "monster spray" during the vacation, but Peter is stunned to learn that Mitzi is the granddaughter of an idolized baseball player known as "Big Apple."
Also, along the way, he invites his closest friend Jimmy Fargo on the trip with him, a privilege gifted to compensate for having to spend a vacation alongside Sheila, but is irritated when Jimmy starts to spend more time with her than with him out of sympathy for her own good friend's inability to join her on the trip too. Along the way, Peter develops a huge infatuation on a teenage librarian named Isobel and Fudge is inspired to author a picture book after learning about Mitzi's own book, "Tell Me a Mitzi."
Frank Fargo, Jimmy's father, a celebrated painter, also receives inspiration after the Hatchers' baby daughter Tootsie toddles across a canvas with blue paint smeared on her feet, commencing a series of paintings appropriately entitled "Baby Feet."