The Herdmans are six scrawny children ( Ralph, Imogene, Leroy, Claude, Ollie and Gladys) who are notorious delinquents in their community. Among other things, they lie, steal, smoke cigars (even the girls!), and hit little kids. The kids basically raise themselves. Their father walked out when the youngest was two years old, and their mother works double shifts at the shoe factory.
The story is narrated by Beth Bradley, a girl in Imogene Herdman's class. Beth's brother, Charlie, unwisely lies to Leroy Herdman, saying that he doesn't mind Leroy stealing his dessert at school because Charlie gets all the snacks he wants at Sunday School. This leads to all six Herdmans showing up at church the next Sunday for the first time in their lives.
Beth's mother is put in charge of the Christmas pageant when the original leader, Mrs. Helen Armstrong, fells and breaks her leg. The announcement for the auditions happens to be on the day the Herdmans show up at church, and, avid movie fans that they are, the Herdman's volunteer (and threaten) their way into all six of the main parts.
In spite of the Herdmans never having heard the Christmas story before (and wanting to change the script so they can hunt down the evil King Herod) and in spite of never making it through a complete rehearsal, the show does go on and turns out to be, as everyone says, the best Christmas pageant they have ever seen. Nobody can put a finger on the precise reason, but it has to do with the fact that the Herdmans just act like real people, probably like the characters in the Bible story really were, rather than the radiant, haloed saints that are usually portrayed.