Holly Tyler, an energetic 16 year-old, moves into her 28 year-old conservative sister Valerie's New York City apartment when their father gets a job in Japan. Everything in Val's neat and orderly life begins to fall apart as soon as Holly comes through the door. Holly likes to cause trouble, but her intentions are almost always good. She forms a close relationship with Val as she matures, picking up some friends and boyfriends along the way, including her best friends Gary and Tina, her past boyfriends Henry and Ben, and her current boyfriend Vince. Val also gains many interesting friends and is involved in many interesting relationships, including her best friend Lauren, her past boyfriends Jeff and Rick, and her current boyfriend Rick.
The girls are trapped in a spa, but Holly and Val commandeer a horse in an attempt to make it to the wedding on time. Meanwhile, Gary and Vince say good-bye to each other, as Vince heads to Chicago for a new job. Vic and Val finally tie the knot, Holly has a surprise visitor, while Lauren, Gary and Tina all find love.