Holly and Valerie are sisters who move in together in New York. 16 year old Holly manages to unravel 28 year old Valerie's orderly life, which includes a promising career in public relations, a handsome boyfriend and her own apartment. Every time Valerie sets a boundary for her sister, Holly leaps right over it breaking something in the process. The one constant is the sisters' affection and concern for each other's happiness and their ability to get out of every jam Holly gets them into.
Two young women push their sisterly relationship to the limits by becoming roommates when their single father takes a job in Japan. Fun-loving 16-year-old Holly Tyler moves in with her straitlaced 28-year-old sister, Valerie Tyler, in New York City.
Holly and Val plan a day of pampering, but Holly finds herself in a "sticky" situation when she sneaks out of a spa without Val's knowledge in order to meet J.C. CHASEZ from the pop group *NSYNC.
When Holly's unintentionally disruptive antics finally become unbearable, Val creates a G.R.C. a Good Roommate Chart for Holly to follow. Afraid of being sent to live with her father in Japan, Holly strictly adheres to the obsessive regulations.
Holly and Val battle for possession of a teddy bear they both cherished when they were younger. When the struggle becomes too divisive, the sisters agree to give the stuffed animal to a little girl in their building.
Val tries to be less of an authority figure to Holly, but Val's patience is tested when Holly and Gary secretly leave for a wild party on Long Island. Worried, Val and Jeff snoop in Holly's e-mail and go after her.
Holly becomes smitten with a young man, Kyle, who is leading a protest movement to retire a zoo's performing parrot. But the situation gets out of control when Kyle is so inspired by Holly's passion for the cause that he kidnaps the parrot.
Holly is excited to form a trio with her longtime pal, Samantha, and current best friend, Gary. When Gary feels neglected because of all the girls' shared memories, Holly advises him to give Samantha a chance.
Holly takes a job at a copy shop and thinks that her new co-worker, Jake, is interested in her. But when she follows Jeff's advice on how to deal with the situation, she becomes the laughingstock of the store.
Determined to start their own Thanksgiving tradition, Val and Holly cancel plans with their relatives, claiming that Holly is sick. However, the girls' happy day goes awry when the family arrives at their apartment to surprise them.
Holly is enamored with a college student, Evan, who frequents Copy That, the photocopy shop where she works. After Evan accidentally takes Holly's cellular telephone instead of his, she calls him and accepts his invitation to a party at his apartment.
Since Val's busy work schedule prevents her from being with Holly and Jeff, Val suggests that the two of them attend a concert, along with Jeff's gorgeous ex-girlfriend Dana.
While Jeff is out of town, Holly takes Val on a night out. However, the female bonding stops when Hunter, a guy Holly wants to date, makes a romantic advance on Val. The situation gets worse when Hunter's jealous girlfriend, Sasha, arrives.
Jeff asks neurotic Val to be more spontaneous. But he regrets making the request when she sneaks into his apartment wearing a sexy cheerleader outfit and Jeff's visiting mother, Linda, arrives unexpectedly, having fought with her husband.
Holly volunteers to work in Val's office during spring break from school. The girls clash over the tactics Val should use to get a promotion over a competitive colleague, Lauren.
On Valentine's Day, Holly thinks that Jeff is planning to propose marriage to Val. Meanwhile, Gary gets a date with a great girl, but the pressures of the holiday overwhelm him, and he insists that Holly come along with her date.
When Holly spends all of her money on an expensive sweater, Jeff agrees to hire her as a waitress, Holly's first job. But he regrets his decision when Holly wreaks havoc in the restaurant and even causes the chef to quit.
When Val returns from a romantic skiing trip, she astounds Holly by announcing that she has ended her relationship with Jeff. Holly tries everything she can to help her sister cope with her feelings, but Val's neuroses soon become unbearable.
Holly and her friend Henry skip school and "borrow" Val's car, but they panic when the car is discovered missing. Meanwhile, Gary helps Jeff gain a new restaurant management job but expects free food in return.
Upon learning that Val has a reputation for being antisocial at work, Holly encourages her sister to mingle with her colleagues. However, an evening out becomes fodder for office gossip when Val finds herself in a compromising position with her boss.
Holly thinks that Val should start dating again and completely disregards her friend Henry's warnings about introducing Val to his older brother, Noah. When Holly finds out that Henry's concerns are valid, she scrambles to get Val to cancel the date.
When Val's vindictive neighbor, Mindy, finds out that Val never got approval for Holly to move in, Mindy persuades the tenant board to evict Holly.
After discovering Holly and her new boyfriend, Henry, alone in Holly's bedroom, Val lectures Holly on the meaning of sex and relationships.