Lesbian couple Nic (Annette Bening) and Jules (Julianne Moore) have been together for twenty years, bringing up two children thanks to the contributions of an anonymous sperm donor.
As 18-year-old daughter Joni (Mia Wasikowska) prepares to leave for college, she and younger brother Laser (Josh Hutcherson) get curious about their heritage.
Making contact with Paul (Mark Ruffalo), the laid-back restaurateur whose sperm bank deposit helped bring them into the world, the two set off a chain reaction that threatens to tear their alternative family apart.