The film starts with a young woman named Bea (Sydney Sweeney) entering a coffee shop, asking the barista if she can go pee, but she is told that the bathroom is for customers only. A nearby customer, Ben (Glen Powell), sees this and pretends that Bea is his wife and that he is buying something for her to help her out. Bea is charmed by him and goes into the bathroom where she accidentally wets her pants because of the sink, so she spends time trying to dry herself before coming out. Before she leaves, Ben calls out to her because she has a toilet seat cover stuck to her shoe, but she thinks he is going to ask her out, so he saves face by doing so.

Ben and Bea spend the day together, hitting it off before going back to his apartment where they share a grilled cheese and sleep together (non-sexually). In the morning, Bea tries to quietly sneak out, but Ben sees her and feels disheartened. Ben’s friend Pete (GaTa) shows up moments later and sees a wrench out, as it means Ben told Bea about his late mother, something he never does with a regular hook-up. Meanwhile, Bea is calling her friend and saying that she might actually like Ben, but as she goes back, she overhears Ben telling Pete that she was a disaster and that he couldn’t get rid of her fast enough, hurting Bea.

Six months later, Ben and Pete meet with Pete’s sister Claudia (Alexandra Shipp) and her girlfriend Halle (Hadley Robinson). Halle introduces her younger sister, who happens to be Bea. Things quickly grow awkward, as both friend groups have heard about their unpleasant encounter. Ben and Bea then proceed to bicker and hate each other.

About a year later, Claudia and Halle have gotten engaged and are having their wedding in Sydney, Australia. This means Ben and Bea have to see each other again. The group arrives and meets Bea and Halle’s parents, Leo (Dermot Mulroney) and Innie (Rachel Griffiths), plus Pete and Claudia’s mom Carol (Michelle Hurd) and their stepdad Roger (Bryan Brown). Leo and Innie expected Bea to get married first and finish her law school practices, unaware that she dropped out. They also bemoan the breakup between Bea and her ex-fiance Jonathan (Darren Barnet)…so they invited him to the wedding to get them back together.

Ben and Bea join Claudia and Halle on the beach, where Ben sees his ex, Pete and Claudia’s cousin Margaret (Charlee Fraser). Claudia can see that Ben still has some lingering feelings for Margaret, but she is there with her new beau…Beau (Joe Davidson).

The group gathers for a dinner party at night. Everything is fine at first until Ben and Bea start arguing, leading to them accidentally lighting a firework that hits a flower display, causing a fire that gets on Halle’s hair.

Fearing that their childish behavior will ruin the wedding, Pete, Roger, Halle, and Claudia all conspire to get Ben and Bea to hook up so that they can get it out of their systems and let the wedding go on without a hitch. The pair quickly catch on to the scheming and swim out to a boat they were asked to pull in, and they discuss their own scheme. Hoping to get her parents off her back with Jonathan, Bea suggests that she and Ben pretend they are actually falling for each other, which would keep Jonathan away and also get Margaret to notice Ben again. He agrees to go along with the plan.

Ben and Bea start to act like they are into each other, beginning when they join their friends on a hike. They attempt to fondle each other, but nobody notices because they see a koala. They then find a spider in Ben’s shorts, causing them to freak out and make Ben take all his clothes off in case there’s more spiders on him. Bea ends up lending Ben her own shorts. Later, Ben bonds with Leo and Innie, showing Leo how to swim based on how his mom taught him. Bea thanks Ben for being honest with her dad.

While Ben and Bea start to get along a little more, Ben spends time with Claudia, discussing how she dumped him, while Bea hangs out with Jonathan around the resort.

The group later goes on a boat ride at night. Ben and Bea try to get everyone’s attention again by doing the “I’m flying” scene from “Titanic” on the ship’s bow. They manage to get people to notice, especially Margaret, but then Bea falls into the water, and Ben jumps in after her. They make it to a dinghy before the group notices them and calls for a rescue. While they are waiting, Ben and Bea have a heart-to-heart, and Ben apologizes for what she heard him say about her. As they are airlifted, Ben starts to show nerves, and Bea starts to sing “Unwritten” by Natasha Bedingfield to him since she overheard him listening to it on the plane ride over, and he calls it his “serenity song” to calm him down while flying.

Ben and Bea are brought back to their hotel, where Ben makes Bea a grilled cheese sandwich like when they first met. They then start to kiss and end up sleeping together, but Ben ends up leaving in the morning before he realizes Bea noticed him, leaving her saddened like he was.

The day before the wedding, Ben and Bea end up coming clean about faking their relationship because they know the others were conspiring over them. Bea’s parents also find out she dropped out of law school. A commotion happens where the family dog knocks over the wedding cake, and Claudia storms out.

On the day of the wedding, Ben overhears Claudia and Halle arguing, making it seem like they might call off the wedding after everything that’s been going on. He goes to Bea’s room and suggests they keep up their good behavior for Claudia and Halle’s sake. Ben and Bea apologize to them, and the two get married successfully. During the reception, Margaret dances with Ben since she said Beau took off. Bea sees Margaret kiss Ben, so she leaves the party heartbroken. Ben realizes he no longer feels the same way about Margaret and learns from Claudia and Halle that Bea went somewhere. Ben then calls for a helicopter ride because he knows where she went.

Ben finds Bea outside the Sydney Opera House since she mentioned wanting to go there when they got stranded in the water. Ben explains what happened with Margaret and admits his true feelings to Bea, who reciprocates, and they kiss.

The two go back to the reception and dance, where the DJ starts to play “Unwritten” with everyone singing to tease Ben. Later, Claudia and Halle admit they faked their fight to get Ben and Bea to make up. Also, Margaret and Jonathan end up together.

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After a seemingly pleasant first date, a misunderstanding leads Ben and Bea to hating each other. Complicating things is them having to fly to Sydney, Australia for the wedding of Bea's sister Halle and Ben's friend Claudia. Their friends plot to get them to hook up since Clauda and Halle fear the two will ruin the wedding with their bickering. When they find that Bea's parents brought her ex-fiance Jonathan to get them back together, along with Claudia's cousin Margaret who used to date Ben, the two decide to go along with everyone's desire for them to get together and pretend they are a couple so that Bea's parents can back off, and Margaret will notice Ben again since he still likes her.

Following some awkward attempts at getting the others to notice them, Ben and Bea begin to develop genuine feelings for one another, but another misunderstanding leads to them fighting again. Ben eventually realizes he doesn't feel the same about Margaret anymore, even after she tries to come onto him, so he finds Bea at the Sydney Opera House, where he admits his feelings. She feels the same, and they kiss.