Oscar Grubman, a 15-year-old portrayed by Aaron Stanford, is a remarkably mature teen who is heading home from school just in time for Thanksgiving. He possesses a talent for speaking French, has a fondness for quoting Voltaire, and finds his peers, particularly girls his age, to be far too superficial and inexperienced. When Miranda Spear (played by Kate Mara), an attractive classmate, expresses interest in him, Oscar tactfully declines her advances. Instead, he reveals to his friend Charlie (Robert Iler) that his heart is set on a more mature woman—his stepmother.
Oscar arrives at the home of his father, Stanley Grubman, a university history professor portrayed by John Ritter, and his stepmother, the passionate cardiologist, Eve, played by Sigourney Weaver. During the evening, the Grubmans host a party where Stanley tries to introduce Oscar to a girl closer to his age, but Oscar turns her down, his gaze fixed on Eve—the object of his affection. When he attempts to express his feelings, Eve remains completely oblivious to his advances. Following Stanley's suggestion to walk the girl home, Oscar instead hails a cab for her.
Feeling dejected after his thwarted attempts, Oscar sneaks out to a bar with a fake ID and consumes alcohol. There, he unexpectedly encounters Diane Lodder (Bebe Neuwirth), Eve's best friend. Observing his inebriated state, she offers him solace at her apartment. As events unfold, Diane gives him a massage, and they end up in a surprising romantic encounter. Awaking the next morning brings an awkward confrontation with Diane's boyfriend, Phil.
Determined to make things right, Oscar concocts a story for Stanley when asked about his whereabouts. He prepares a surprise lunch for Eve, bringing her newfound perspective on love to the table—shouldn't the liver be the symbol of affection instead of the heart? However, their uplifting discussion is cut short by a phone call from Stanley, who casually mentions Diane’s attendance for dinner. Anxious about the possible revelation of his secret rendezvous, Oscar seeks out Diane at a tea room, where he finds her surrounded by friends who all seem to be in on the prior night’s escapade.
During dinner, tensions rise as Diane flirts playfully with Oscar, even engaging in a game of footsie. After excusing herself, Oscar follows her, leading to a kiss that doesn’t go unnoticed by Stanley. Diane ultimately confesses their relationship to both Stanley and Eve, prompting Eve to scold Diane for seducing someone so young. In the wake of this chaos, Oscar faces Eve in a competitive game of tennis where exchanged barbs quickly escalate; he ends up hit by a tennis ball, inadvertently confessing that his actions with Diane were merely the result of drunkenness and Eve's scarf.
This leads to a pivotal moment where Eve begins to grasp the depth of his feelings for her, culminating in a heated kiss that leaves them both flustered. As Thanksgiving break comes to an end, Eve and Stanley accompany Oscar to the train station. They exchange lighthearted banter about his "liver," which he claims is sore but intact. On the train, he reunites with Charlie and unexpectedly sees Miranda again. As she references Voltaire with a wistful gaze, Oscar reflects on his previous obsession with Eve, admitting to Charlie that it wasn’t as significant as it once felt. Oscar’s smile as the train moves forward reveals a subtle shift in his perspective on love and maturity.
