Malcolm, whose frenetic home life is a kind of zoo, begins the fourth season by accompanying his family to the real thing. A visit to the zoo offers close encounters with creatures great and small. But Hal wishes he could have passed on an encounter with Lois's former beau, who lectures on rain-forest animals. Elsewhere, Reese butts heads with a goat; and Malcolm and Dewey take a tumble into a tiger den. Another storyline checks in on ne'er-do-well Francis, now the unlikely foreman of a dude ranch owned by a cheery German businessman.

Malcolm suffers extreme humiliation in his first week as a high-school freshman. Francis surprises everyone by becoming the responsible one at work and Dewey revels in being a latchkey kid.

Christopher Lloyd adds to his gallery of eccentric characters, playing Hal's quirky, cranky and very wealthy father, Walter, in this rollicking episode. The occasion is a birthday celebration for the old man, but the snooty female in-laws seem more interested in offending Lois. Their shabby behavior initially goes unnoticed by Hal, who's desperately trying to bond with his self-centered dad; and by Dewey and Reese, who seek some of Grampa's dough by currying favor with him. But when Lois is reduced to tears after being excluded from a group photo, the family rallies to her side with an all-out counterattack that turns the party into pandemonium.

Hal hires a steamroller and becomes obsessed with crushing things, while Malcolm puts on an act to attract the attention of a non-intellectual girl.

Malcolm is grounded on his birthday once again when his vengeful battle with Reese gets out of hand. Hal sets out to buy a comic for his son, but has an unfortunate run-in with the shop assistants.

Malcolm dates a girl whose father dislikes him. Francis becomes chief mediator in a dispute between Otto and his cowboy neighbors, and Dewey discovers a boy who looks just like him.

After joining the basketball team at school, Malcolm soon finds himself under the coach's skin due to his constant complaining, which eventually gets him kicked off the team. Determined to put an end to his horrible habit of talking before he thinks, Malcolm starts "holding his tongue" which solves all the problems with his coach, girlfriend, and parents, but it ends up causing more stress than before. Meanwhile, Hal takes up speed walking and is desperate to be the best while Reese gets Craig to take him and Allison to a concert and almost ruins his date.

Hal and the boys are destined for disappointment as they visit Francis and Piama at the ranch, where they assume plenty of mischief will be planned - but are saddened to discover their brother is now a responsible grown up.

Cloris Leachman returns in her Emmy-nominated role as the spiteful and irascible Grandma Ida. She's even nastier than usual (if that's possible) because of a tumble down the front steps that resulted in a broken collarbone, prompting her to sue her daughter and son-in-law for pain and suffering. And there's more momentous news on the home front: Lois is pregnant. So she pleads with Ida to reconsider her lawsuit: "We're already stretched to the limit financially as it is. Obviously, a new baby is going to make things even harder on us". "Yes, yes, of course", Ida concurs. "You should settle".

After being rattled by a battle among the boys, a pregnant Lois ponders the consequences of raising another brawling kid. So she fantasizes that her unruly sons have morphed into well-behaved young daughters. Malcolm becomes Mallory, Reese is transformed into Renee and Dewey turns into Daisy.

A sex lecture from Lois rattles Malcolm; a ranch calamity unnerves Francis; disharmony among Hal's music group is a prelude to their concert. Also, a day in jail tests a defiant Reese, who faces a "Scared Straight" encounter.

After Lois is out of town, Malcolm and Hal got into a fight leading Malcolm to get expelled from his house. Malcolm lives with his girlfriend in her attic, as Hal put signs for "Missing Malcolm".

Jessica, the boys' sixteen year old babysitter, tricks them into doing chores around the house, even Malcolm is fooled. Hal gets a job as a salesman.

Malcolm tries to emancipate Dabney from his overbearing mother while Hal invites his ne'er-do-well old buddy to the house. At the ranch, Francis is forced to share a room with Otto.

A garage sale that's entrusted to Reese (yikes!) yields treasures near and dear, like Hal's old radio transmitter, which he makes operational. And a PC dating back to 1976 that Malcolm plans to sell to Craig for 00.

An academic competition faces the Krelboynes in an event Malcolm derides. Especially when he learns his teacher has a cheating plan. Also: a commitment to a formal dance ruffles Reese, who's now trying to ditch his date.

Clips from past episodes are interwoven with the current squabbling of Hal and Lois, who set out to draw up a will and wind up at odds over their legacies and their kids. As Hal tells Lois, "We are not equipped to deal with these issues".

When Hal and Lois go out of town for a "last chance" getaway before the baby comes, Reese has a big bash that gets out of hand. College-age thugs show up, cancel the party and set up a drug-making operation in the garage. All Reese and Malcolm can do is knuckle under. The good news is, Dewey's not around. The bad news is, his baby sitter is Lois's annoying co-worker Craig (David Anthony Higgins). Meanwhile, Hal and his very pregnant wife are off at a romantic retreat - feuding.

Leonard (Jason Alexander), a cynical, genius chess player with no friends or job, makes Malcolm think he could grow into a similar adulthood. Determined to prevent this undesirable fate, Malcolm tries to help Leonard change his life by lining him up for a job interview with the obnoxious Craig. Meanwhile, Francis finds employ as a nude model and begins to delight in the exposure.

Hal and the boys are at a bridal expo, leaving a very pregnant Lois at home with Francis. They get two surprises: an unexpected visit from Grandma Ida (guest star Cloris Leachman) and Lois' water breaking.

While Lois is in labor, the house is hectic, and Hal, desperate to get home, has crashed the car on the way out of the expo.

Hal and Lois search in vain for a day care to take their newest arrival but find nothing. Until they stumble upon an idea that's just inspired. Spiritually inspired it seems as the family joins a local church for the free co-op day care service.