Lois and Hal leave the boys alone for the weekend with Francis. the boys assume that if all goes well, Francis may not have to go back to military school.

The whole family attends a Krelboyne family picnic, and Malcolm dreads his performance in the event's Academic Circus will make him look like a freak. Disasters abound as stage acts bomb, families feud, and Malcolm's dad serves meat to vegetarians.

The boys stay home from school when their mother thinks it's Sunday instead of Monday. Malcolm thinks of a plan to get Francis out of trouble once again. Hal gets sucked in by a Porsche dealership.

Shenanigans abound at the fancy spot where the family's dining with the Kenarbans, and at Francis' school, where rowdy teenage girls are partying.

Hal feels like his job is a piece of c* after going to Dewey's class for parent job day. So he decides to take a "kidney failure" weeks off. While Francis is forced to work at the lucky aid over spring break.

Lois forbids the boys to ride a motorbike they found, but Reese disobeys and breaks a leg.

A negative job-performance review riles Lois; hoops play with the boys energizes Hal; a vermin infestation busies rat-killer Francis.

Highway horror: Reese gets a learner's permit, which he promptly proceeds to abuse by commandeering a driving-school vehicle.

Dewey rules, thanks to his new dog that intimidates Malcolm and Reese; Hal and Lois hit the town with big plans---and flu symptoms.

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.

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.

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.

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 is convinced to play Softball by Lois and ends up doing very badly at it due to Lois's hold on him. Meanwhile, Francis tries to pick fights with Lois, Reese tries to break Francis's record of collecting toilet seats from a local junkyard, and Hal accidentally applies to become a member of the CIA.

The Buseys disappear after Dewey leaves the special-needs class; Hal becomes the leader of a group of brainless bodybuilders.

Lois thinks Jamie is out to get her when Jamie knocks over a shelf and nearly crushes her.

Malcolm discovers a man, Norm, living in the Lucky Aide. He agrees to keep Norm's secret as long as he provides Malcolm with tidbits on his new crush, a fellow Lucky Aide employee. Meanwhile, when Reese takes a job as an exterminator, he quickly learns that to drum up business he must infest the neighbors' lawns, and chooses caterpillars as his weapon. But instead of spreading the caterpillars, he has a change of heart and begins to nurture them, ultimately leading to his own menagerie of butterflies.

After Lois and Hal are reminded of the tight budget they must uphold, Hal discovers one of his boys has been dialing a 900 number. His attempt to explain the error to the hotline's billing department turns awry when he forgets to hang up the phone, incurring a whopping $800 charge. Meanwhile, Reese takes a job at a research clinic that pays him to pop experimental pills, and a reluctant Malcolm takes on a mortifying job at the Lucky Aide as a costumed Uncle Sam on stilts.

The family attends the counterculture Burning Man festival in the desert.

Malcolm receives a check for $10,000. After Hal and Lois find out, they decide to keep the money and spend it on something for themselves. However, when Dewey overhears this, he wants to get in on it, too. Now, they'll have to find a way to pay Malcolm back.

Reese meets a cute vegetarian named Carrie when he goes back to work at the meat plant. At first, he goes along with her political talk, but she catches him with a pork chop. So, to get Carrie back, he sets free all the cows at the meat plant. Malcolm, meanwhile, tries to fool Lois in order to attend a rock concert.

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.