The arrival of Francis's new bride on Hal's birthday sparks a fray that drives the younger siblings out---and into a posh hotel.

Clever Hal invigorates Dewey's soccer team; sly Reese blackmails a philandering neighbor; obsessed Malcolm battles a computer program.

Girls in the house spell trouble for the boys, who are on their own while Dad's hosting a poker game that's more about one-upmanship than cards.

Back in the general school population, the Krelboyne whiz kids wind up in cliques with jocks, skateboarders and brooding "Goths."

New monkey business occupies Reese, now a self-appointed law enforcer; a capuchin monkey has it in for Lois's co-worker, Craig.

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.

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.

Acting dumb like Reese eases Malcolm's stress and lands him a date with the new light of his life, an attractive girl who's really a dim bulb.

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.

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.

Grandma Ida is 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.

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.

Leonard, 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.

Lois goes into labor while Hal and the boys are at a bridal exposition. Ida moves in after her apartment burns down and she refuses to stay with Susan.Dewey reveals the date his parents chose for the baby's planned delivery is his birthday in an announcement at the exposition to humiliate Hal.

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.

When Dewey's pet rabbit does good in a state contest, the family must travel to Las Vegas for the nationals. But Reese unknowingly sells the rabbit to a restaurant. Now it is up to Dewey (with a little help from a "choking" Reese) to try and save the rabbit.

Meanwhile, Malcolm tells Hal and Lois that they embarras him, and that is why he didn't tell them he won state science honors. As a form of repayment, Malcolm gets great seats for him and Lois to see Lois' favorite singer. Lois then gets lucky and is invited backstage where all she really wants to do is figure out why Malcolm doesn't want her around.

Hal, on the other hand, is looking all across Las Vegas for a slot machine that he has had dreams of winning the jackpot on.

Francis is also in trouble when he gets behind on duties at the ranch, and for the first time sees that Otto isn't just a pushover for everything.

Malcolm skips out on Thanksgiving to go to a party; Reese becomes abusive to Hal and Dewey as he attempts to prepare Thanksgiving dinner.

Malcolm chooses Reese as his subject for an Abnormal Psychology mid-term project; Dewey earns some extra cash as a street performer.

Hal, Reese, Malcolm, and Dewey go into business selling Christmas trees, with the boys using some of their own money, but two priests send homeless people to their business, trying to sabotage their sale.

Lois blows her top over a hidden 15 percent surcharge on the bill at her family's favorite pizza place and stages a boycott...but Hal and the boys are sneaking slices on the side. Malcolm meets Angela and becomes instantly smitten with her intellectual family...but starts to feel like a heel because he's asked another girl to the school dance and is forced to break it off with her--er--them. And Francis is reluctant to let Otto hire an assistant for him...but soon comes to like the idea.

Hal sets up Abe Kenarben on a blind date with Jaime's babysitter Polly but then realizes Lois has also set her up on a blind date - with Craig. Malcolm, Dewey (with his "lucky" shirt) and Reese also meet Reggie Jackson and Francis is embarrassed by Piama in front of his co-workers.

When Dewey's annual candy selling event for school comes up, he uses the excuse of "these bars help support america" to sell them. Malcolm and Stevie attempt a science experiment, and are shocked when Reese begs to help.

Reese's new girlfriend Beth is about as dumb as he is. Malcolm initially dislikes her but they soon get to like each other too much and Beth breaks up with Reese. Devastated, Reese runs away and joins the army under the false surname.

Reese continues to be a great soldier in the Army until he finds himself in a situation where his Sergeant cannot help. But after comparing being captured to being grounded, he takes action and leads his team to victory. But being such a successful soldier has it's bad parts as well, as Reese prepares to jump from the plane - right into Afghanistan. Malcolm has his hands full as well as he attempts to prove Hal innocent. But being good with numbers helps as Malcolm notices all of the prosecutions given dates are on Fridays - which Hal hasn't worked on in over 15 years. And with Reese missing and Hal's case, Lois falls deeper and deeper into insanity.

Hal and Lois, strapped for cash, decide that Christmas gifts will have to be homemade this year. When everyone makes much better gifts than Hal, he spins a bigger and bigger lie about the present he has in store for them. In desperation, Hal pick-pockets Malcolm's secret credit card, and buys the family a ski vacation.

When Francis and Piama choose to celebrate Francis' 21st birthday at the house, Hal is reminded of a promise he made to his son as a young child, involving a motorcycle trip that Lois quickly forbids. Hal and Francis have no choice but to sneak off for their wild adventure, leaving their women behind. Meanwhile Malcolm, Dewey and Reese are held captive in their own home by a bully.

Francis receives a visit from Lois and Hal to celebrate his one-year anniversary of sobriety. However, when they attend one of his A.A. meetings, it's revealed that it was Lois who forced him to start drinking. Back home, Dewey finds Hal's spare key for his car and tries to get Malcolm and Reese to drive him to the arcade, but, of course, they refuse.

Malcolm struggles with his speech when he's elected valedictorian of his graduating class at school. However, Lois refuses to listen to any of his complaints and reveals that she wants him to become President of the United States. Meanwhile, Reese does everything he can to get a permanent job as a high school janitor, which includes creating the most disgusting mess ever. In the meantime, Hal has problems with Malcolm's Harvard tuition, while Francis finally gets a real 9-5 job that he actually enjoys. Also returning for the family celebrations is Grandma Ida, as they all say goodbye to Malcolm when he leaves for college.