Six months after ending their engagement, Ali Leighton Meester reminisces about their time together as Christmas approaches. As she bakes, she laments giving up her ex-fiancé's loving family, while Graham Robbie Amell works nonstop as a game developer, and his boss moves up the work deadline to Christmas, forcing him to tell his family he cannot leave LA.
Graham travels to Minnesota as a surprise, only to find Ali there. Sheepishly, his mother Jeannie Kathryn Greenwood and sister Mindy Veronika Slowikowska reveal that they invited Ali and gave her his childhood bedroom. He learns that his father Dennis Michael Hitchcock and brother Elliot Steven Huy have also stayed in touch with her. Graham decides to stay and try to cast Ali as the villain to force his family to choose him over her. Realizing his plan, Ali makes a bet with him to see who can win over the family. The two then engage in childish sabotage.
At the Stroop Christmas party, Ali meets Brady Thomas Cadrot, Graham's old high school rival who now works for Dennis. Meanwhile, Graham hits it off with Jess Donna Benedicto, a local. Later that night they all go to a bar where Ali and Graham try to make each other jealous. In the midst of their hijinks, Mindy reveals that she has broken up with her girlfriend, Heather, so Graham and Ali both comfort her.
The next day, the competition continues as the family prepares for Christmas Eve dinner. Graham sabotages Ali by using up all the butter, so she finds a jar of butter to use for her cookies instead. At dinner, Brady and Jess unexpectedly join. Graham and Jess refuse to eat dessert as everyone enjoys Ali's cookies—only for the group to realize that she used THC butter. While everyone else is stoned, Graham accidentally reveals Mindy's breakup to his parents, dampening the mood.
Graham drunkenly stumbles into his childhood bed, waking Ali up. The two begin arguing which culminates in a kiss. The next morning, they wake up cuddling only for Jeannie to barge into the room with breakfast. As Ali hides under the covers, Jeannie gives Graham advice about Ali which both take to heart. At church, Graham attempts to talk to Ali about the prior night and says that he's willing to forgive her for dumping him. She retorts that perhaps he is the one that needs her forgiveness.
As the family head to the local ice rink for Christmas hockey together, Dennis suddenly has a heart attack. Graham immediately administers CPR, saving his life. At the hospital while Dennis recovers, Ali and Graham have a truce and wish each other the best.
The next morning Ali says a tearful farewell to the whole family and heads to the airport. Finally realizing he can't let her go, Graham races to stop her only to find her back on the doorstep. After giving an impassioned speech, the pair finally get back together.
A year later the family celebrate Christmas in LA at Graham and Ali's new house. The pair are engaged once more, Graham has his own business and Ali finally gets her food truck.
In a post-credits scene, two years have gone by and the family is celebrating Christmas in LA once again. Hearing a knock at the door, Mindy and Graham, who is carrying his baby, go to open it only to see Heather on their doorstep. Graham reveals that he invited her as revenge.
