Abby Holland (Kristen Stewart) and Harper Caldwell (Mackenzie Davis) live together in Pittsburgh and have been dating for a year. Everything is going well, they are spending the Christmas holidays with Harper's parents and Abby has even planned to propose to her girlfriend on Christmas morning.
But contrary to expectations, Harper hasn't even come out to her parents yet, so she wants them to keep their relationship a secret and Abby to just pretend to be a roommate.
Abby understands that Harper is worried about coming out because her parents have particularly high expectations of their children and, as a result, a rivalry has developed between the sisters. But the more Abby learns about Harper, the more she worries about whether Harper is serious about the relationship.