Overattentive single father Jim (Will Ferrell) is taken aback when he learns that his daughter, Jenni (Geraldine Viswanathan), has become engaged to her boyfriend Oliver (Stony Blyden). Struggling to grasp the reality that his little girl has matured into a woman capable of making her own decisions, Jim expresses disbelief and concern over Jenni’s youth and the fact that Oliver did not seek his permission before proposing and planning to cohabitate. This discontent leads to a fiery confrontation with Jenni, which eventually helps Jim come to terms with the relationship.
In his quest to finalize wedding plans, Jim attempts to book a small inn located on Palmetto Island, Georgia, the same venue where he and Jenni's mother were once married prior to her passing from cancer. Tragically, the owner of the inn, Scarlett (Martha B. Knighton), accepts his reservation for June 1 but unexpectedly passes away, leaving her calendar unfilled. Meanwhile, Jenni is surrounded by her friends, including Heather (Keyla Monterroso Mejia), Lexi (Zamani Wilder), and Kelly (Ramona Young), while Oliver counts on his friends Josh (Joshue Charles) and Sam (Matt Mercurio) for support.
In a parallel situation, television producer Margot (Reese Witherspoon) discovers that her sister Neve (Meredith Hagner) is set to marry her boyfriend Dixon (Jimmy Tatro), who is an exotic dancer. Neve struggles with their mother’s disapproval of her life choices, while Margot, who shares a strained relationship with her family, finds solace primarily in her bond with Neve. Margot volunteers to plan the wedding, opting once again for Palmetto Island, where she and Neve made childhood memories visiting their grandmother. The new manager of Palmetto House, Leslie (Jack McBrayer), unwittingly accepts both bookings for the same date—June 1.
As the wedding weekend approaches, both groups converge on the island, only to discover the unfortunate double booking. Jim presents evidence that the reservation was made under Scarlett’s guidance, verifying the booking with a pen mark found on the calendar; however, officially, the reservation is secured under Margot’s credit card. It becomes clear that Jenni’s wedding planner, Heather, has not conducted any site visits, while Margot has actively checked the venue three times. The impossibility of hosting two weddings on a small island leads Margot, initially claiming the venue for Neve and Dixon, to agree on a compromise. They will share the space by alternating the use of indoor and outdoor venues, as Margot is touched by Jim's heartfelt story about being a single dad and baking a wedding cake for Jenni. It is agreed that they will share the sunset, allowing Jenni to get married first, followed by Neve.
In a surprising turn of events, Neve confides in Margot that she is pregnant, insisting that Margot keep this news under wraps to spare their traditional mother Flora (Celia Weston) from distress. The family dynamics are complicated further by the presence of their brother Colton (Rory Scovel) and sister Gwyneth (Leanne Morgan), increasing the tension surrounding the wedding. Margot ends up dismissing Pastor Luther (Nick Jonas), a decision that doesn't sit well with family friend Pastor Jerry (Wesley Mann).
During the rehearsal dinners, a rainstorm forces both groups indoors, leading to numerous humorous and tense interactions. Jim makes an impression on Margot’s family members, but this frustrates Margot, who struggles with feelings of inadequacy due to her family’s critical nature. Jim and Jenni further complicate things by performing Islands in the Stream, raising eyebrows among their guests due to its suggestive undertones. A catastrophic incident occurs when Margot, in a drunken state, accidentally bonks Jenni with a microphone as she attempts to interrupt her speech.
Conflict arises when Jim overhears Margot disparaging Jenni’s closeness with him, igniting a plan of petty revenge where Jim intentionally drags out Jenni and Oliver's ceremony to prevent Neve and Dixon from marrying at sunset. This scheme involves a chaotic failed boat stunt that leaves the wedding party soaking wet. Although Jim and Margot share a moment later in the evening where apologies are exchanged, Jim feels guilty about the actions he took earlier. Margot retaliates by rallying her siblings to disrupt Jenni and Oliver's reception.
The ensuing events reveal deeper familial issues, as Margot and her siblings confront their personal struggles while observing what they thought was Oliver’s unfaithfulness. Margot discovers that the man she thought was Oliver was actually a groomsman and struggles with the chaos that ensues—leading Jenni to ultimately abandon the island as Neve and Dixon finally manage to complete their ceremony.
An emotional confrontation occurs at the reception, resulting in a public showdown between Margot and her mother that lays bare several family secrets. An unexpected encounter with an alligator adds to the madness, leaving Margot hurt until Dixon, a trained medic, comes to her aid, opening the door for reconciliation.
As the dust settles, Jim learns through social media that Jenni and Oliver have annulled their marriage. He decides to ride back to Atlanta with Margot, intent on convincing her to revisit the possibility of a relationship. Their conversation leads to a heartfelt connection as Jim pledges transparency in the future. Although Jenni and Oliver decide against rekindling their marriage, they plan to relocate to Memphis together.
In the end, Jenni shares with Margot that Jim has feelings for her. The two prepare to meet again, sealing their budding romance with a kiss before Margot departs. As Thanksgiving rolls around, Margot’s family, accompanied by Jim, Jenni, and Oliver, pay a visit to her in Los Angeles, culminating in a family photo that symbolizes unity. In a whimsical moment, Jim proposes to Margot, and she proposes eloping instead, hinting at their continued adventures and misadventures in love.
