In the vibrant city of Boston, we meet Ally Darling, a woman navigating her tumultuous 30s. After her boyfriend, Rick, breaks up with her following her request for him to attend her sister Daisy's wedding, Ally is left in a vulnerable state. To make matters worse, she soon loses her job, leaving her feeling adrift. While riding the subway home, she stumbles upon a thought-provoking article in Marie Claire titled "What's Your Number?" that suggests women with more than 20 past romances are less likely to find their ideal partner.
This revelation pushes Ally to take stock of her own romantic history, only to discover that she has been involved with 19 men. Determined to change her love life, she swears off romantic encounters until she can find the one who will truly make her heart race. However, after a wild evening at Daisy's bachelorette party, she wakes up to an unexpected complication: a hangover and the realization that she has once again crossed the line with her former boss, Roger.
To dodge the impending embarrassment of facing Roger, Ally invites her neighbor, Colin Shea, a musician with his own issues, to assist her in getting rid of him. Unbeknownst to Ally, Colin is at her door for his own reasons—trying to shed a one-night stand who is beginning to take things too seriously.
As fate intervenes, Ally bumps into her past love, Disgusting Donald, who has transformed from the awkward, overweight guy she once knew into a confident and attractive man about to tie the knot. Disheartened by her own romantic misadventures, Ally resolves to contact her ex-boyfriends, hoping one of them will have matured into the man she dreams of marrying. She ropes in Colin to help with this quest, trading his digging skills for a place to crash after his escapades.
Among her exes, one stands out: Jake Adams, whom Ally perceives as her "one that got away." However, as she reconnects with each of her past lovers, the encounters are filled with awkwardness and disappointments that reveal exactly why each relationship failed—ranging from one ex being stuck in a dead-end job to another wanting her to fake a British accent.
Feeling despondent after these failed reconnections, Ally's spirits are lifted when she enjoys a night out with Colin, leading to a burgeoning connection between them. However, just as things appear to be blossoming, Ally hesitates, yet still chooses Colin as her date for Daisy's wedding.
With whispers about Colin's past romantic escapades spreading among their circle, doubts loom over any potential romance between him and Ally. The tension escalates when Ally discovers that Colin had secured Jake's contact info without revealing it to her, stoking feelings of betrayal and jealousy between them.
Amidst this turmoil, Ally reignites her romance with Jake and invites him to her sister's wedding. As the night unfolds, Colin performs with his band at another wedding, allowing Ally to savor joyful moments dancing with her father. However, during a dance with Jake, she confesses her past romances, which horrifies him, leading her to realize that her heart belongs elsewhere.
In a determined search for Colin at various weddings across the city, Ally finally admits her feelings to him, receiving an equally heartfelt response. The story culminates with Ally waking up next to Colin, only to receive an exuberant call from an old flame, confirming that Colin is indeed her 20th and final romance, closing a chapter in her life and opening a new one filled with love and connection.
