The family goes to the zoo. Malcolm is going through a crisis, and he and Dewey find themselves in a unfortunate situation. Reese angers an animal at the zoo. Hal is jealous of a former flame of Lois's who works at the zoo.

In a desperate attempt to get rid of his Krelboyne past and start afresh, Malcolm ends up making things even worse than they were.

Malcolm stops thinking in order to go out with a very dumb girl. Hal wins some money, and rents a steamroller.

Malcolm sneaks around to date a girl whose father hates him; Hal and Lois become more productive when they can't have sex for a week; Dewey discovers he's an "evil twin"; Francis mediates a dispute between Otto and his cowboy neighbors.

On a visit to Francis' dude ranch Hal can't help being disappointed in Francis' success, Reese and Malcolm fear Francis has turned into an overly responsible grownup, and Gretchen punishes Dewey for breaking a valuable doll.

When Lois' mother visits, she trips on a leaf and decides to sue Hal and Lois...just as Lois finds out she's pregnant again.

Lois fantasies what her life would be like if she'd had girls instead of boys.

Lois uses a long drive with Malcolm to further his sex education; Reese spends a day in prison; Dewey plants seeds of discontent in Hal's vocal group; Francis has an accident branding Otto's favorite cow.

Hal lands a second job at an electronics store, while Malcolm is dumbfounded by their new babysitter, who's the same age as himself.

Hal spends a wild night with an old high school buddy; Malcolm tries to liberate Dabney from his mother's control; Francis spends the night with Otto when their wives are out of town.

The family decides to hold a garage sale to fund repairs of the bedroom wall and Lois puts Reese in charge. Hal finds his old radio-transmitter and starts running a pirate broadcast like he once operated in college.

As Lois and Hal write their wills in preparation for the baby's birth, they recall various colorful incidents with their four current children.

When Hal and Lois go on a less-than-romantic getaway, Reese throws a party at home, which is crashed by delinquent friends of Francis, who take over the garage and hold the boys hostage.

Hal takes the boys to a bridal show; Lois' mother wants to move in with the family; Malcolm gets an offer to enroll in a prestigious private school.

Lois continues with her strenuous labor, as friends and family all panic around her.

The family joins a church in order to take advantage of the free day care; Francis concocts a UFO story to help the dude ranch.