Nic (Annette Bening) and Jules (Julianne Moore) form a loving lesbian couple, each having welcomed a child into their lives through the same anonymous sperm donor. As an obstetrician, Nic serves as the family’s main provider and assumes a more disciplined role, while Jules, who is embarking on a new venture in landscape design after previous career setbacks, takes on a more relaxed approach in parenting. Although their family life seems harmonious at first, the romance between Nic and Jules starts to feel stagnant over time.
Their younger son, Laser, played by Josh Hutcherson, is keen to discover the identity of his biological father but must wait until he turns 18. In an act of boldness, he persuades his 18-year-old sister, Joni (Mia Wasikowska), to reach out to the sperm bank for help in meeting their biodad, Paul (Mark Ruffalo). Their initial encounter is surprisingly pleasant, with Joni captivated by Paul’s carefree, bohemian lifestyle, and he excited about being part of their lives. Despite Joni’s request for confidentiality to protect their mother’s feelings, the truth soon comes out, leading to an invite for Paul to dinner, where the atmosphere is awkward yet friendly.
During dinner, Jules mentions her landscape design business, prompting Paul to hire her for work on his garden, a decision that doesn’t sit well with Nic. As Jules begins to receive the appreciation she craves from Paul, in stark contrast to Nic’s lack of support, she soon crosses boundaries by impulsively kissing him, resulting in a passionate affair. Their relationship intensifies, and Jules begins to spend increasing amounts of time with Paul.
Tension arises when Nic feels undermined by Paul, particularly after he grants Joni freedoms she has explicitly forbidden, such as taking a motorcycle ride with him. In an attempt to rectify the growing friction, Nic suggests a dinner at Paul’s home, hoping to stabilize the situation. While the evening starts positively as Nic finds common ground with Paul through their shared love for Joni Mitchell's music, she quickly becomes disturbed when she finds strands of Jules's red hair in his bathroom and bedroom.
Once home, a confrontation ensues between Nic and Jules. Initially, Jules denies the affair but ultimately confesses, causing deep emotional pain for Nic. Despite Jules asserting that she’s not in love with Paul and hasn’t abandoned her identity, the home becomes increasingly tense, with Jules opting to sleep on the couch and the children feeling betrayed by both her and Paul. Paul, clearly enamored with Jules, suggests she leave Nic and move in with him, an idea Jules ultimately rejects.
On the eve of Joni's departure for college, Paul unexpectedly appears. An angry Nic confronts him, accusing him of intruding on their family, and advises him to start his own. This confrontation forces Jules to gather her family, where she acknowledges the difficulties of marriage, tearfully admits her mistakes, and pleads for their forgiveness. The following day, the family, pointedly absent of Paul, takes Joni to college. As Nic and Jules emotionally say goodbye to Joni, sharing a gentle moment of connection, their son Laser innocently remarks that they shouldn't split up, claiming they're "too old" for such decisions. Jules and Nic share a laugh, and the film concludes with a poignant moment—hand in hand, they drive home together, hopeful for their future.
