Megan (Keira Knightley) is an aimless twenty-eight-year-old living in Seattle, Washington who is in a committed relationship with her high school sweetheart and still close with her high school friends. At her friend Allison’s (Ellie Kemper) wedding, her boyfriend unexpectedly proposes. She also sees her father cheating on her mother. Shocked and confused, she flees and runs into teenager Annika (Chloë Grace Moretz) and her friends outside a grocery store. After buying them alcohol, Megan plays with their skateboard. When they ask her to join them on their night out, she does.
Returning home, Megan’s boyfriend suggests that rather than having a big wedding, they elope. Megan agrees, but delays the wedding by pretending she has a week-long career advice seminar to attend.
Megan receives a call from Annika asking her to pretend to be her absentee mother at a guidance counsellor’s meeting. Megan goes to the meeting, then asks Annika if, in return, she can stay at her house.
Annika is unsuccessful at sneaking Megan into her house, where her single-parent lawyer father Craig (Sam Rockwell) questions Megan about her relationship with his daughter. Megan lies that there was a gap between her lease expiring and her move into a new apartment. Craig reluctantly allows her to stay and gradually begins to trust her. Annika asks Megan to drive her to her estranged mother’s house to see her for the first time in years.
Craig asks Megan out for a drink. When they return—having had more drinks than they planned—they end up having sex. The next morning, he offers to let her stay longer than the week she asked for, but she declines.
Annika, seeing them kiss, tentatively tells Megan that she approves of the relationship. She then finds the engagement ring in Megan’s purse. Megan tells her she’s engaged, and that Craig has no idea. On the way home from shopping for Annika’s prom dress, they begin arguing, and Patrick is distracted and crashes the car. He tells Megan he has been drinking the wine she bought for them because he is upset that his parents are sending him to boarding school. Feeling guilty, Megan tells the police she was the one driving the car. She still has alcohol on her breath from that morning, and is arrested. When Craig arrives to free her, Megan reveals her engagement and that she bought alcohol for his daughter.
Megan’s father picks her up at the police station. He says he has confessed his cheating to her mother and they are trying to work things out.
Misty shows up at Megan’s door with the white dress Megan bought and a few other things she’d left in Patrick’s car, and tries to convince Megan to go to the prom. When Anthony comes to the door, they tell him Misty is a girl Megan was mentoring at the seminar. Once he goes back upstairs, Misty offers Megan a prom ticket and urges her to go for Annika’s benefit.
Megan continues to lie to her fiancé about where she was for the past week. At the airport, he takes a selfie of them and sends it to their high school friends. Megan, realizing their relationship is stifling her, breaks up with him, unfriends her high school friends, and goes to Annika’s prom, where she tells Annika to be honest and pursue Junior, a friend with whom she is in love. Megan goes to Craig’s house and leaves him a box of wine with a note that says, ‘Can we try again? Check [Yes] or [No]”. She knocks, then hides at the bottom of the steps. Craig answers the door, reads the note, sees Megan at the bottom of the steps, and closes the door. Not to be deterred, Megan knocks again, then picks up the house key hidden outside—but before she can unlock the door, Craig opens it again and lets her in.