Eloise McGarry finds herself attending the wedding of her longest friend, Francie Millner. Initially chosen as the maid of honor, she had to withdraw two months prior after her boyfriend, Teddy — Francie's brother and the best man — broke up with her via text message. Teddy has since reconciled with his former girlfriend, Nikki, who took Eloise's place as maid of honor. During the reception, Eloise ends up at Table 19, feeling disheartened and uncertain about staying. Just as she's about to leave, she encounters Huck, who tries to comfort her with advice that doesn't sit well.
Deciding to remain, Eloise meets her table companions: Jerry and Bina Kepp, a married couple who run a diner chain and are Facebook connections of the groom's father; Renzo Eckberg, a high-schooler hoping to meet someone new; Jo Flanagan, the childhood nanny of the bride; and Walter Thimble, the bride's cousin on parole. They enter into a debate about whether Table 19 is a "good table." Eloise shares her insight that she was involved in planning the wedding and knows this table is meant for "guests who should have known not to show up."
As the newlyweds arrive and begin their first dance, Eloise, feeling out of place, accepts a dance from Huck, leading to an unexpected kiss. When Huck declines to sit with her, she realizes he’s not just a guest but crashing the wedding using a false name. Initially at a loss for conversation, the group bonds after Jo discovers that Eloise is pregnant. They confront Teddy, learning that he ended their relationship after Eloise expressed doubt about their parenting capabilities. Hurt and feeling undervalued, the group leaves to smoke some pot and discuss their reasons for being at the wedding.
Walter reveals he’s a former criminal who stole $125,000 from the bride's father, while Renzo shares that his prom coincides with the wedding, but he came in search of a romantic connection. Jo feels neglected by the kids she nannied, which sparks a tender moment among the group. Amidst the introspection, Jerry offers Renzo dating tips, and Eloise uncovers the truth about Jo's health — she’s fighting cancer and predicts they'll have babies around the same time. Tensions rise when Jerry and Bina have an argument about their failing marriage, prompting Bina to go back inside with Walter.
Taking Jo's approach, the group sets out to find the wedding crasher Eloise met earlier, enlisting the help of the wedding photographer and hotel staff. They discover that "Huck" is actually the groom from another wedding, leading Walter to steal Huck’s wedding cake as revenge for destroying their own earlier.
Back at the celebration, the bride and groom express their gratitude for all attendees, with Francie reaching out to Eloise and apologizing for the seating arrangement, prompting Eloise to contemplate leaving early. However, Francie's nostalgic story about Teddy inspires Table 19 to view him differently, leading to a push for Eloise to confront her feelings. As the core wedding party departs on a ferry, Eloise calls out an apology to Teddy from the dock, but her words go unheard.
Later, Eloise, Walter, Jo, and Renzo return for one last slice of cake, while the Kepps discuss the state of their marriage and find reconciliation. The atmosphere shifts dramatically when Teddy emerges, soaked from jumping off the boat to reach the shore, and asks Eloise to repeat her previous words. In a heartfelt moment, they agree to forgive each other and embrace. Renzo requests the band to play another song, prompting everyone, including Teddy, to join in the dancing.
Months later, with Jo having succumbed to cancer, the Kepps are happy and tending to her dog, while Renzo is dating someone new, and Walter refers to Table 19 as his family. Eloise and Teddy are together, welcoming a baby into their lives. They send out birth announcements to their friends, including all the guests from Table 19, revealing they have named their son Joe.
