Dee Dee attempts to get closer to Mona by planning a joint birthday party, reminding Mona of bad memories from their childhood. Meanwhile, Dee Dee unwittingly reveals that Phyllis lied to Mona about things that have shaped her world view since she was young; and, as a birthday gift, Phyllis finally tells Mona the truth.

Dee Dee persuades Mona to attend a banquet that's being held in honor of their father, but the experience reminds Mona that she's the odd one out within her dad's ""new"" family.

Dee Dee invites Mona to a ""girls night"" with her friends, but Mona instantly feels out of place amidst the ritzy women. When Dee Dee gives Mona an expensive purse, Mona deduces that her half-sister is trying to make her over. Meanwhile, Spencer is attracted to one of Dee Dee's friends.

Spencer thinks that he has gotten a girl pregant and Dee Dee and Mona force him to get an AIDS test.

Mona drags Dee Dee to a self-defense class, where she accidentally hurts her little sister while practicing a back flip. Overwhelmed with guilt, she waits on Dee Dee hand and foot, unaware that Dee Dee's milking the injury.

The gals' already tenuous friendship spirals toward dissolution when Dee Dee celebrates the end of her school year by hosting a raucous celebration that keeps Mona awake all night. Things get worse when Dee Dee installs a hot tub, which floods Mona's apartment. Stepping in to mediate, their father, Charles, reminds them that ""like it or not, we're family."" But the squabbling continues, until a medical emergency involving their dad arises.

After Dee Dee loses her old room to the baby and Big Dee Dee misses several appointments, Dee Dee begins to worry that the baby is forcing her out of her parents' life. She suffers from so much stress that she has a breakdown and hears a doll talking to her. Mona recommends that she make an appointment with Phyllis to discuss her problem. She says that she feels as though she is the only one who is worried about the way the baby will change her life. Phyllis assures her that her reaction is perfectly natural and that she should air her concerns with Big Dee Dee. Before she gets around to it, Big Dee Dee admits to her own anxieties about the child's birth, and thanks Dee Dee for her support. The conversation reassures Dee Dee about her importance in her mother's life. Meanwhile, Spencer continually cancels on Mona because of plans with Camille. She looks to Adam to take his place, but Adam isn't too thrilled about this.

Hoping to prove herself to her father, Dee Dee goes against his wishes and tries to personally handle a tenant who has failed to pay rent for the last few months. She must come up with a new plan of attack after the man retaliates by threatening to expose several building code violations involving Dee Dee's apartment. Meanwhile, Mona reluctantly takes her mother out on the town. She feels crowded when Phyllis begins showing up regularly at her hangout and playing matchmaker for the patrons.

Dee Dee organizes a singles mixer at a local club. She bristles at Mona's claim that she only dates a certain type of man (well-groomed and upwardly mobile), and tries to prove her wrong by picking up a tough guy named Keno. She takes off with him on his motorcycle and joins him at a bar, where she rides a mechanical bull. She has a good time exploring this new side of herself--until she runs into a very angry woman who claims to be Keno's girlfriend. Mona and Nick crash the mixer and meet up with his ex-girlfriend, Raquel. Mona is disturbed to discover that Raquel is her dead ringer, and fears that Nick is just with her because of the resemblance. Meanwhile, Dee Dee must actually care for Drew herself after firing the nanny. She is mortified when she must turn to Phyllis for help when her son will not stop crying.

Dee Dee vies with her law school rival, Stephanie Seymour, for a summer internship. Dee Dee believes that she is the more qualified candidate; but Stephanie sucks up to the man in charge, Mr. Washington. She also tries to use her sex appeal to her advantage, so Dee Dee retaliates by showing up for her final interview in a revealing outfit. She gets the job, but Mr. Washington's comments and behavior prompt her to fear that she got the job for the wrong reasons--and that her new boss might want something in return. Meanwhile, Kai develops a crush on Spencer and asks Mona to pass along this information to him. Spencer is thrilled, and decides to get involved with Kai and try to use her for career advancement. However, he panics when she starts talking about a relationship during their first date.

Mona returns from a Hawaiian vacation and clears the air with Spencer. They reaffirm their decision to just be friends. While looking through some vacation photos with her family, Mona reveals that she can now ride a horse. She is stunned to learn that Dee Dee still has the horse their father gave her as a child, Daddy's Favorite, and that she has lied for years about having not kept the horse. (The horse led to a major rift between Charles and Phyllis that separated the sisters for a decade.) Mona decides to buy her own horse, but it doesn't quite go as planned. Meanwhile, Adam helps Spencer uncover the cause of Kai's sudden unpleasant attitude toward him.

Dee Dee is furious to discover that Mona has become friends with Dee Dee's rival, Stephanie Seymour, after meeting her through a work event. Dee Dee insists that Mona should dislike the same people that she does because it is her duty as a sister. Mona tricks the duo into having tea with her in the hopes of getting them to make peace, but things do not exactly go the way she had planned. Meanwhile, a rejected applicant for the talent search refuses to take no for an answer.

After Dee Dee bests her at an online word game, Mona grows frustrated by the fact that she always loses to her sister. She becomes determined to beat her at something, and sees her opportunity when Dee Dee organizes a poker party for one of her law school professors. Their mothers decide to crash the event, where Phyllis winds up winning Big Dee Dee's wedding ring in the pot. Meanwhile, Adam tries to get access to a copy of Spencer's personal music playlist.