The Smeds and the Smoos did not like each other because they were different colors. The Smeds were red and the Smoos were blue, and they thought being different was a reason not to get along.
When their families are sleeping, Janet and Bill sneak out to play and have adventures together.
When they cannot find Janet and Bill, the families feel worried and upset. They search for their children everywhere.
The main message of the story is that differences in appearance should not prevent people from being friends and loving each other. It teaches acceptance and understanding.