Charlie is a well-to-do bachelor with a house at the beach, a Jaguar in the front, and an easy way with women. His casual Malibu lifestyle is interrupted when his tightly wound brother Alan, who's facing a divorce, and his son Jake, come to live with him. Together, these two and a half men confront the challenges of growing up; finally. Complicating matters are the brothers' self-obsessed, controlling mother, Evelyn, Alan's estranged wife, Judith and Charlie's crazy neighbor Rose, who wants to be a part of his life and is willing to do anything to be around. After the death of his brother, Alan Harper meets and befriends a lonely young man named Walden Schmidt who turns out to be a billionaire. Unable to afford his brother's home, Alan sells Walden the house, and as a way of showing his gratitude, Walden allows Alan and his son Jake to move in with him.
Charlie returns to his bachelor lifestyle after Alan moves out of the house. Alan suffers a breakdown after his divorce from Kandi.
Charlie finally tells Alan the truth about why he did not marry Mia. Charlie and Alan deal with a noisy neighbour.
Charlie tries to surf to impress Dottie, a beautiful woman he just met, but after he nearly drowns he is convinced that he saw his late father.
Alan tries to hire his ex-wife's lawyer to help him win custody over the dog he and Kandi shared.
Alan is overjoyed by the prospect of Judith remarrying, as he would no longer have to pay alimony. Alan gets Charlie to help him when it looks like the marriage may be derailed due to Jake’s dislike of his mother’s fiancé.
Charlie is dating a woman whose personality and life are like his mother’s.
Berta's daughter, Naomi, is pregnant and Berta drags Charlie with her when she confronts the baby’s father. Alan reveals a secret fetish.
Alan suffers from insomnia. He visits his psychiatrist (Jane Lynch), who reveals that the cause of Alan's sleepless nights is his jealousy towards Charlie, and his inability to glide through life.
Charlie gets tired of Alan's mooching and reaches the breaking point during the brothers' double date. Jake is left home alone for the first time.
Charlie must choose between his girlfriend and his housekeeper; Alan looks to sell Kandi's condo, but sex keeps getting in the way.
Charlie's friends and family put a crimp in his plan for a romantic Christmas Eve date. When Evelyn sees who the date is, she goes to great lengths to force a wedge between them.
Berta discovers that Alan has been dating her daughter (Sara Rue).
When Jake tells Charlie and Alan that he often overhears Judith and Herb (Ryan Stiles) making out, it triggers a repressed memory for Charlie.
Charlie has second thoughts after setting Alan up with their new neighbor (Brooke Shields)..
Rose is moving to London, and Charlie is surprised by his reaction to the news. Alan lies to a woman he met online.
Charlie's young, hot girlfriend takes him to a new club. She invites Alan to meet one of her friends but the men are shocked to learn that they are considered as being "old".
After Evelyn tells Alan and Charlie how she feels about there being no mourners at her friend’s funeral, the boys share their feelings that this could also be the case at her funeral. Evelyn tries to prove them wrong by introducing them to a couple she help adopt a boy from China. The boys return fire by inviting the young man to a play date with Jake.
Alan tries to bond with Jake when he realizes that his childhood is almost over. Kandi pursues a career in acting.
Charlie takes a liking to Myra, the witty sister of Judith's fiancé, Herb.
Alan looks forward to Judith's wedding, while Charlie and Myra continue their romance.
Alan and Charlie re-examine their sexual identities after Alan befriends Greg, a gay man. Meanwhile, Jake has a crush on Greg's daughter.
Jake gets lucky after Charlie takes him to the racetrack. Meanwhile, Alan has a difficult time at the DMV.
Charlie is shocked when the handyman working on his house beats him out for Chloe's affections. Alan has no sympathy for Charlie’s distress.