As the magical hour of October 31st, 1998 envelops Cornell University, Tom Bailey Jr. (dressed as Bill Clinton) sneaks into the dorm room of Monica, his designated date. Instead of finding her, he encounters her quirky roommate, Hannah. What starts as an awkward meeting soon evolves into a flirtatious interaction, but Tom's patronizing comments quickly dampen the mood. After their brief encounter, the two part ways, only to reunite ten years later in New York City. At this point, Tom has struck gold with his "coffee collar" invention, making a significant profit from every cup sold, while Hannah dedicates herself to her career at the Metropolitan Museum of Art, remaining one of Tom's closest friends since college.
As they navigate the turbulent waters of adulthood, Tom revels in his carefree lifestyle, filled with fleeting hookups, contrasting sharply with Hannah's commitment to her professional and personal development. Their friendship deepens during a family wedding for Tom, where a shared dance ignites new feelings between them. However, Tom fails to recognize these feelings beyond friendship.
When Hannah reveals she will soon depart for Scotland on a work assignment lasting six weeks, Tom experiences an awakening to the emptiness of his transient relationships and acknowledges that he does not want to lose Hannah. In a moment of self-reflection, he resolves to confess his feelings upon her return.
However, Hannah's homecoming comes with a twist. She brings with her a fiancé, Colin, a wealthy Scottish Duke. Tom, eager to win her heart, agrees to be her maid of honor, deliberating on how to prevent the impending wedding. As he becomes embroiled in the wedding preparations, he also has to deal with Melissa, Hannah's cousin and past love interest, who harbors resentment and complicates matters by convincing Tom to invite an unexpected guest—a sex toy salesman—to the bridal shower, straining his relationship with Hannah.
Undeterred by the chaos, Tom leans on the support of friends who provide him with a "How To Be A Perfect M-O-H 101" guide. His moment of redemption comes when he dazzles at the wedding with a memorable plate-juggling act that astonishes Hannah. Yet, just as they reconnect, she drops news of her plans to move to Scotland after tying the knot, leaving Tom contemplating the implications for their relationship.
Feeling defeated but resolute, Tom makes a bold decision to follow Hannah to Scotland. Once they arrive at Eilean Donan Castle, Colin's family invites Tom to join the Highland Games competition. In this high-stakes challenge, Tom hopes to best Colin but ultimately falls short. During the rehearsal dinner, a revelation strikes Hannah that every dish served has been personally hunted by Colin, leading to further contemplation about her choices.
When it’s time for Hannah's bachelorette party, she takes part in a Scottish tradition of kissing male patrons at the pub for a fee. In a daring move, Tom takes advantage of the moment and steals a passionate kiss from Hannah, which she returns. However, when Melissa tries to seduce Tom just when Hannah seeks to discuss the kiss, his endeavors seem thwarted. This makes for an emotionally charged moment in which Tom chases after Hannah, but sadly, she rebuffs him, remaining committed to Colin.
Devastated at the thought of losing Hannah forever, Tom vacates the celebration early, but as the sun rises the next morning, clarity strikes him. He decides he must act fast to stop the wedding. With no available ferry, Tom commandeers a horse and races to the ceremony, arriving just in time to storm the church and declare his love for Hannah. To his relief, she reciprocates, and they together halt the wedding, offering apologies to Colin, who reacts by punching Tom in the face.
As the dust settles, Tom and Hannah feel liberated. They ride off into the sunset, making their way back to New York, where they ultimately exchange vows atop a rooftop beneath the twinkling stars, ushering in a promising new chapter of their lives united as partners.
