NOTE: This spoiler was submitted by Jeremy
It's New Year's Eve, sometime in the early 90's. Five high school friends are out on the town recording their night, having fun and enjoying their time together. They sneak into a hotel pool and go skinny dipping.
Ten years later, we catch up with one of those friends, Megan (Keira Knightley). She's 28 years old with an advanced degree, yet she works for her dad Ed (Jeff Garlin) as the sign girl for his tax advice business. Ed loves his daughter but he is too easy on her in regards to her low ambitions. She lives with her high school sweetheart boyfriend Anthony (Mark Webber), who is a relatively successful photographer.
Megan meets up with her three best friends, Allison (Ellie Kemper), Savannah (Sara Coates), and Danielle (Kirsten deLohr Helland) for a little get-together in anticipation for Allison's wedding. They check out the venue for the bridal shower. There is a big Buddha statue in the place, and Megan tweaks the nipples on the statue. The ladies sit together to play a bridal shower game, which gets awkward when Megan uses Allison's fiance's name in a sexual context. Allison is upset by this and also by Megan tweaking the statue's nipples.
Megan and Anthony go to Allison's wedding a few nights later. Megan tries to resist laughing at the corny wedding vows and the even cornier dance that the newlyweds do. The music later plays for everyone to dance. Megan and Anthony slow dance, and he takes the chance to begin his proposal to Megan. He only kneels before Megan panics and pulls him back up on his feet, saying it would be inappropriate to do this on Allison's night, but Anthony assures her that he ran it by Allison and she was excited for them. Allison then comes over and asks Megan to find Allison's mother. Megan does so and ends up finding her dad outside making out with Allison's mom, and she sticks her hand down her pants. Horrified, Megan runs back into the party and makes an excuse to go.
She retreats to the parking lot of a supermarket, where she is approached by a teenage girl named Annika (Chloe Grace Moretz). She goes up to Megan and asks her if she can buy her and her friends beer if they give her money. Annika's friends also chime in to ask for coolers and wine. Megan buys it all for them, and some fruit for herself. She hands the kids their stuff, and then notices the skateboard of Annika's friend Patrick (Dylan Arnold). She flips it with her foot, to the amusement of the kids. She is about to leave, but the kids want her to hang out. They end up throwing toilet paper at someone's house and going to the park to drink all the booze. Megan drunkenly talks to one of the kids, Misty (Kaitlyn Dever), about how she feels left out of her group after the awkwardness at the bridal shower party. Misty tells her to not make this a sad conversation since they notice Patrick looking sad since his parents are splitting up and his dad is being a dick about it. Megan then goes over to Annika, who is talking to the third friend, Junior (Daniel Zovatto). After Junior leaves, Annika tells Megan that her mom left her and her dad years ago. She gives Megan a disposable phone, which is when Megan realizes how late it is and she leaves.
Upon returning home, Anthony finds Megan and asks what happened to her. She makes an excuse about how she drank too much and got sick and had to leave. He then brings up the "kneeling" and figures she may not be ready for something so big, but he suggests that they can go to Vegas and elope. Megan says yes, but she tells him to wait a week because she claims to be going to a self-help seminar in Orcas Island. Anthony supports her and agrees to wait a little longer.
The couple goes to a brunch held by Allison. She is upset with Megan for leaving early since she missed the group photos and other things. Megan spots her dad and retreats to the bathroom. Ed finds her and she tells him that she saw him the night before. Ashamed, Ed tries to explain that he and Megan's mother have just grown apart as a couple. Megan leaves and rejoins Anthony to talk to Savannah and her husband. They want Megan and Anthony to be the godparents to their baby. They accept. Savannah then says they want to name the baby Jupiter, but with two P's, since nobody else will have that name.
While driving away to her fake trip, Megan gets a call from Annika, who asks her to go to her school and pretend to be her mom for a parent-teacher conference, with the promise that she'll owe her. Megan does so and poses as Annika's mom. They meet with Annika's teacher, who thinks Annika is not too focused on her future, though Megan says that it isn't necessary to make some big plan for her future just yet.
Since Annika owes Megan, Megan decides to use this opportunity to lay low for the week. Annika brings her over to her house. They watch a make-up tutorial as Megan does Annika's eyes, when her dad Craig (Sam Rockwell) barges into her room. Annika says Megan is a new friend and they are going to the movies.
The ladies really go to a party. There, Annika sees a girl jump on Junior, to her dismay. Megan goes off and hangs out with the kids, listening to music and piercing one boy's ear. She finds Annika reading a catalog that has her mom Bethany (Gretchen Mol) modeling. This, along with what she saw with Junior, has made Annika unhappy that night. Megan knows she likes him, and she tries to cheer her up.
Megan sleeps over Annika's home until Craig barges in again. He tells Megan to go downstairs so he can do a little interview on her. Megan makes up a story that she is waiting to get a new apartment, among other things. Craig thinks she is okay, despite knowing she is a grown woman hanging out with teenagers, and he lets her sleep in the guest room.
Annika asks Megan to take her to see her mother. They drive to Bethany's home, where she seems less than happy to see Annika. She goes to fix her and Megan lemonade, but she takes a while in the kitchen. Megan goes to talk to Bethany, who admits that she wanted to be the cool mom for Annika, but she knew she couldn't be, so she left. Megan tells her that all Annika wants is for Bethany to seem interested in her daughter's life. Bethany goes into the living room with Annika and gives her some new lingerie. When they leave, Annika asks Megan to join her and her friends at prom.
Megan treats Annika to a choccachino and a muffin at the nearest coffee shop, when Allison spots Megan. Megan goes to talk to her, and Allison is none too pleased to find out that Megan lied, since Anthony said Megan was going on her trip. She also missed the birth of Savannah's baby (which they named Calisto since someone else named their kid Jupiter).
At night, Misty comes over for a sleepover. Megan goes downstairs to talk to Craig. He is distressed and in need of a drink. The two go to a bar and get drunk off shots. They end the night by making out and later sleeping together.
The next morning, Craig makes a drink to cure his hangover. Megan goes into the kitchen and they start kissing. Annika sees this. Later, while she, Megan, and Misty are waiting for Patrick and Junior, Annika tells Megan that she saw her kissing her dad. Megan apologizes and says she didn't count on falling for her dad. The boys arrive, with Patrick driving and chugging orange juice.
The group goes to the mall to shop for prom clothes. While the ladies try on their clothes, Annika goes through Megan's purse to look for gum and finds the engagement ring from Craig, with an engraving that says "A&M Forever", which Megan didn't even know about. Annika is pissed and tells Megan she's worse than her mother.
On the drive home, Megan tries to talk to Annika, who doesn't want to hear it. Junior intervenes, but Megan cuts him off, making Annika more mad. Patrick yells at them to stop arguing as he drives, and as he takes his eyes off the road, he crashes into a mailbox. He admits to Megan that he was drunk off the wine that she bought for him. The police show up, and Megan takes the fall for Patrick. She fails the breathalyzer test since she drank some gin with Craig earlier that day, and she gets arrested.
Craig goes to bail Megan out. Out of guilt, she confesses to Craig that she's engaged and that she bought the kids alcohol, and that she took Annika to see her mother. Craig leaves her, feeling hurt by the lies. Ed later goes to pick Megan up, where he admits that he did a stupid thing by cheating.
Megan returns home to Anthony, who is happy to see her. Not long after, Misty shows up at the door to give Megan her prom ticket, imploring her to come.
Megan and Anthony head to the airport later on for their elopement, until Megan feels another moment of guilt. She tells Anthony that they can't be together and that who they were as a couple, along with her friendship with the girls, is in the past. Anthony is left heartbroken and devastated.
Megan goes to prom, which is held on a boat. She finds Annika sitting by herself, watching Junior dance with a girl. Megan encourages her to go with her gut and make her own choices. Annika approaches Junior as he is dancing, and she cuts in to say that she likes him and wants to be his girlfriend. Junior says yes and they start dancing. Megan smiles at them and leaves.
Megan goes back to Annika's house to knock on the door and leave a wine box with a note on it for Craig. He opens the door to see that it's asking for another chance, with "yes and no" boxes to be checked off. Craig leaves the note and goes back inside. Megan finds a spare key and tries to go inside until Craig opens the door again. She admits that she fell in love with him. He invites her inside, and he grabs the wine box before closing the door.