Charlie runs into an ex-girlfriend, Chrissy (Rena Sofer), who has a son named Chuck who looks, dresses, and acts disturbingly like Charlie. Alan takes great pleasure making fun of the situation. Charlie learns that condoms are only 98% effective and, thinking that Chuck is his son, he pays Chrissy a large lump sum and promises to send her a check every month for Chuck's maintenance. Just after Charlie leaves Chrissy's apartment, it is revealed that she is only Chuck's babysitter, his name is actually Jeremy, and her plan to extort money from Charlie has succeeded.

Alan lends Charlie $38, which Charlie has no interest in repaying. Alan seeks revenge to get the money back, and pumps out gas from Charlie's Mercedes to his own, but still claims Charlie owes him $24.78 as there wasn't enough petrol in the tank to amortize the full $38. Charlie accuses Alan of being cheap, ends up angrily throwing him out of the house, so Alan and Jake temporarily stay with a friendly Herb and angry Judith. But after Herb bonds with Charlie and Alan crosses a line with Judith, she takes measures to reset the situation, forcing Alan to apologize to Charlie and forcing Charlie to take him back. However, after she and Herb leave, Charlie locks Alan out of the house. Meanwhile, Evelyn offers Jake his first job.

Charlie seeks to broaden his horizons and improve his sex life by taking up cooking, but his focus on egg recipes has an unexpected result. Alan and Jake try to emulate Charlie's lifestyle: Alan by dating two women at once, and Jake by getting drunk, but they find out the hard way that Charlie's way of life is not for them. Jake ends up vomiting violently with Charlie at his side. Alan's first date finds out he has another woman, because of his inability to lie convincingly, and when he tries the "honesty is the best policy" approach with the other woman, he ends up getting Tasered.

While Alan's car is in the shop, Charlie grudgingly drives Alan to work, but he cheers up when he meets Alan's receptionist Melissa (Kelly Stables). Alan begs Charlie not to make a move on her, because she is very good at her job and for a low wage, and Alan is convinced that it would end badly for himself. Of course, that merely spurs Charlie on. Alan's misgivings prove correct when Charlie breaks up with Melissa after they have spent a long weekend together, mostly in Charlie's bed. Melissa is so angry, and so dangerous, as she drives them to work, that Alan feels he has to give into her demands for a raise, health insurance, and a paid vacation.

First appearance of: Melissa

The guys bump into Jake's former teacher Delores Pasternak (now played by Alicia Witt, before played by Missi Pyle) who tells them that the way Charlie dumped her caused her to have a nervous breakdown, lose her job as a teacher and become a stripper. Guilt-stricken, Charlie tries to help her with a room at his house and a job tutoring her ex-student Jake, but later he finds out she has also become strictly religious. Charlie's solution is the same as the first time.

Herb tells Charlie and Alan that he and Judith have been having marital problems. Charlie offers Herb advice on how to stand up for himself, resulting in Judith kicking Herb out of the house. Alan, overjoyed by the situation, goes to Judith's house pretending that he has to pick up Jake's book, and confronts her crying. He hugs her and calms her down, resulting in the two of them having sex. Meanwhile, Jake has been sent to Charlie's for three months, and Herb bonds with Charlie.

Alan tries to escape Judith's grasp as Charlie and Herb go on the prowl to pick up women. They later end up going back to Herb's hotel room with two prostitutes, where Herb declares his love for Judith and races over to win her back. Alan and Judith begin arguing and decide to end their "relationship" when Herb turns up. Later on in the episode, Jake reveals that Judith has become pregnant - leaving Alan completely shocked; Charlie explains this to Jake that Alan's doing math in his head. Meanwhile, to cover up Alan's escapade with Judith, Charlie screws with Jake by leading him to believe he's being sent to military school.

Charlie's new girlfriend Chelsea (Jennifer Taylor) asks him to spend the night at her place, which puts Charlie well outside of his comfort zone. During the night, he finds a lot of problems and cannot sleep. He gets up to leave but steps on Chelsea's cat which screams loudly, waking Chelsea up and causing her to angrily kicking Charlie out. Charlie talks her into giving him another chance, but when Charlie makes too many adjustments she decides she'll happily spend nights at his place from now on. Meanwhile, a pregnant Judith has grounded Jake for mooning a bunch of girls in school and Alan has difficulty keeping Jake obeying the rules of his punishment.

First appearance of: Chelsea

Jake disappears with Celeste (Tinashe Kachingwe), the daughter of Charlie's new neighbor Jerome, a former NFL player (Michael Clarke Duncan), and Charlie is left to cover his back. Charlie talks to Jerome and tells him not to be so overprotective of his daughter, and the two become friends. Meanwhile, Alan scores a hot date (Bellamy Young) with the help of Charlie's Mercedes. He takes her back to Evelyn's house as Evelyn is away on holiday, and ends up being tricked into wearing one of her dresses as the woman steals Alan's wallet and Charlie's car. Worse, Alan is put on hold when he tries to phone the police. Even worse, Evelyn returns from vacation and sees Alan in her dress.

Evelyn offers to pay for Jake's college education, and Alan becomes listless since he no longer needs to work or do anything productive with his life. Charlie later comes up with the idea of Alan paying him rent, but the number that gives Alan a say in what goes on in the house is too high for him. Jake discovers a hidden talent in the culinary arts that may allow him to bypass college altogether, but it ends badly when Jake burns the food and the fire brigade come to Evelyn's house to extinguish the flames.

One of Charlie's old partners in crime, Andy (Emilio Estevez), dies while talking to Charlie, leaving him to reevaluate his own reckless lifestyle and tell everyone in his life (except Alan) how much he cares about them. At the funeral, Charlie begins to worry about his own life and Alan tells him there has always been one person in his life on whom he can depend on: Alan! Just kidding, it's Rose, who Charlie almost proposes to before finding out he's physically fine and returning to his old ways. Berta reveals to Alan that Charlie revised his will to leave her everything if he dies, and tells a horrified Alan that she'll be nice and give him extra time to leave the house before she'll have him evicted then.

Temptation is the theme of this episode as Charlie and Jake fight for the affections of a waitress, Janine (Emily Rose), while Alan and his receptionist Melissa (Kelly Stables), who has since recovered from her disastrous fling with Charlie, fight to suppress their mutual attraction and have to deal with Melissa's loopy mother (Carol Kane).

Judith is pregnant with a baby girl and Alan is convinced that he is the father; he helps Herb paint the baby's room, but gets thrown out after he upsets Judith. Jake is in a bad mood and Charlie thinks he knows the reason.

Convinced that Alan's family doesn't fully appreciate him after they throw him a pathetic 40th birthday party, Melissa invites Alan to live with her and her mom, whose disturbing hobbies quickly send Alan back to whence he came.

Charlie has a relationship problem with Chelsea, who thinks he is still his old self. Charlie panics and suggests couples counseling with Dr. Freeman, which she accepts, while Alan plays their problems to his benefit. Jake is tired of Charlie stealing his dessert and decides to mark what's his.

Charlie must remove a half-naked woman named Wanda (Diora Baird) from his bedroom before Chelsea returns from her ex-mother-in-law's funeral; Jake seems to recognize Wanda but doesn't realize that she is a pin-up girl on one of his posters. Meanwhile, Alan won't let Jake drive the car.

Charlie, to his own surprise, tells Chelsea he loves her, but he doesn't get the response he wanted. After a visit to Dr. Freeman fails to help, Charlie decides he must take drastic measures to restore the 'balance of power'.

Chelsea becomes ill and Charlie must look after her. He is not thrilled about it but he does try, and ends up taking advice and help from Alan.

Charlie, Alan, Gordon, Herb, and Jerome (Michael Clarke Duncan) reminisce about sexual experiences after Chelsea finds some photos on Charlie's phone; Alan's receptionist Melissa is still punishing him for having sex with her mother, and Charlie resolves to fix things with Chelsea because he doesn't want to end up like any of these guys. Meanwhile, Jake goes out with Jerome's daughter Celeste again.

Charlie's desperate to make sure Chelsea and Evelyn don't befriend each other following their first meeting; Evelyn bribes Alan with a waterproof Rolex and Jake with a pizza so they can't help Charlie turn Chelsea against her; Charlie ultimately turns the tables and gets his revenge by befriending Chelsea's not-very-tolerant mother Martha (Meagen Fay).

Alan lets Chelsea fix him up on a blind date with one of her friends. The friend turns about to be Rose, shocking both Alan and Charlie. Things actually go well, but Rose, still smitten about Charlie, reverts to her old stalker self and Alan ends up with one of his model cars glued to his testicles.

Charlie and Alan both face lifestyle changes when Chelsea moves into the house: Charlie is annoyed with Chelsea changing things, and Alan is annoyed at having her spare stuff in his room.

Charlie takes advantage of Alan and Chelsea's fast friendship by having Alan do all of the activities that Chelsea likes and Charlie hates; Alan takes up ventriloquism and pushes Charlie to his limit when he "text-blocks" Charlie while he is in bed with Chelsea, causing Charlie and Chelsea to have an argument.

The return of Mia (Emmanuelle Vaugier) finds Charlie questioning his devotion to Chelsea; the guys rush a laboring Judith to the hospital; Alan reunites with Melissa (Kelly Stables), who now works as a hospital clerk, and struggles in his attempt to write a screenplay; Judith gives birth to Jake's baby sister, but the parentage is still unknown; Herb breaks his leg shortly after Judith breaks her water; Charlie runs into Mia in the coffee shop, resulting in a cliff-hanger season finale.