In the quaint suburban backdrop of Chicago, 16-year-old Samantha "Sam" Baker finds herself on the brink of a significant transition as she eagerly anticipates her birthday—a moment she hopes will be memorable. Unfortunately, her excitement is quickly overshadowed when she discovers that her family has completely forgotten about this important event, preoccupied instead with the upcoming wedding of her stunning, yet self-absorbed, older sister, Ginny.
As Sam navigates the often treacherous waters of high school social life, she stumbles upon an anonymous friend's sex quiz, which inadvertently exposes her longstanding crush on the dashing senior Jake Ryan. Meanwhile, Jake is dealing with his own romantic dilemmas, expressing to his confidant Rock his disappointment with his girlfriend Caroline, who is engrossed in a reckless party lifestyle.
During her bus ride home from school, Sam becomes the target of persistent advances from the nerdy freshman Ted, who appears determined to win her heart. However, her day takes a turn for the worse when she learns that she will be relegated to sleeping on her grandparents' couch, which is overcrowded with wedding guests, including the quirky foreign exchange student Long Duk Dong.
Confronted with the painful reality of being overlooked not only by her family but also by her grandparents, Sam is disheartened further by the feeling that even they have forgotten the significance of her day. In a last-ditch effort to salvage her evening, they send Dong to escort her to the school dance, where he quickly captures the eye of the intimidating Marlene.
While Sam yearns for Jake from a distance, Ted tries to make an impression on his peers Bryce and Wease by dancing with her. However, a misguided high-stakes bet involving floppy disks and underwear leaves Sam crushed and embarrassed. After this humiliating incident, she confides in Ted about her family's negligence and her unreciprocated feelings for Jake.
Offering her supportive advice, Ted encourages Sam to confront Jake directly. Initially hesitant, she ultimately resolves to follow his counsel. Just as she is about to approach him, Ted shares that Jake has asked about her at the dance, leaving Sam both astonished and intrigued. Meanwhile, Jake exits the dance with Caroline, throwing Sam into a whirlwind of confusion about whether Jake truly harbors feelings for her or if she is merely a fleeting infatuation.
Amidst the turmoil of unreturned affection and forgotten birthdays, Sam needs to find the bravery to express her feelings and carve a fresh path for herself. While the festivities at Caroline's party continue, Jake finds himself retreating to his room, grappling with his disillusionment as he attempts to reach out to Sam, but his call is met with imposed disapproval from her grandparents. His frustration escalates as he comes across Ted, trapped under a coffee table, the consequences of the night’s outrageous exploits evident all around them.
As Ted reveals Sam's interest in Jake, he confesses to Ted that his feelings for Caroline have also waned. Seizing the moment, Jake strikes a deal with Ted: he’ll give him a pair of Sam’s underwear in exchange for driving the intoxicated Caroline home in a luxurious Rolls-Royce Corniche owned by his father. Ted’s attempt to impress his friends by stopping for photographic proof of his adventure, however, leads to disappointment when only a brief glimpse of him is caught on camera.
In the meantime, Sam's father takes a moment to apologize for forgetting her birthday and offers sage advice: if Jake doesn’t recognize how wonderful she is, then he isn’t worth her time. As Sam reflects on her situation while lying on the couch, she remains unaware that Jake is also mulling over his feelings for her.
The dawn of a new day brings a sense of reckoning. Sam's mother expresses her regret for the oversight as the family prepares for the wedding. Unbeknownst to Sam, Jake has arrived at her house, misled by Dong's hangover into thinking Sam is on the verge of getting married. Witnessing Caroline and Ted share an embrace in his father's dilapidated car finally pushes Jake to end things with Caroline.
That night, Jake surprises Sam with a beautifully decorated birthday cake featuring sixteen sparkling candles, along with her misplaced underwear. Just as she reveals that her wish has already been fulfilled, they share a heartfelt kiss, culminating in a fairy-tale moment as the credits roll.
