Tom, a middle-aged lawyer living in New Jersey, visits his mother with his wife Allison and their three children. While there, Tom shares with his mother that Allison, currently a stay-at-home mom, is planning on going back to school and starting a career. Allison overhears the conversation and joins in after putting the kids to sleep. Tom disapproves of his wife's decision, worried that he will not be able to take care of the kids on his own while she is away, but his wife insists that it is time to start living her life.

Some time later, Tom returns with his family to see his mother for her birthday, and he takes his daughter to a stream where he used to play with his father when he was a child. Tom tells his daughter about the tiny organisms living in the water, life that exists though the human eye can't see it. Once inside, the family gives Tom's mom, known as Nanoo, her gifts, at which point Allison hands her an envelope. Tom, unaware that Allison had gotten his mom another present, feels confused and betrayed by the gesture. The envelope presumably contains Allison's acceptance letters to graduate school. On the drive back home, Tom and Allison argue about the gift, Tom angry that he was not the first to know.

Allison has been accepted into both Stanford and Harvard to study history. Eventually she chooses Harvard because of the closer distance, which Tom is thankful for. Tom quickly finds an apartment for his wife in Boston, but Allison dislikes it and chooses her own place instead, closer to Cambridge. When Tom goes with Allison to help her move in at the end of the summer, he sees the single bed that she has bought for her apartment and becomes nostalgic, remembering the times they shared a single bed at the beginning of their marriage.

Tom slowly adapts to taking care of the kids on his own, but he remains resentful toward Allison for leaving. One day, when he tries to call her, Tom discovers that her telephone has been disconnected. After a few days of not hearing from his wife, Tom decides to drive to her apartment and confront her, but when he arrives she is not home, though her car is parked outside. Allison returns to find her husband snooping around suspiciously, and though he stays the night, Tom never confronts her as he had planned. Allison's phone remains disconnected, but she calls her family from a pay phone once a week.

For Christmas, Allison and Tom go with the kids to stay with their grandmother. Allison asks Tom's mother, Nanoo, about her late husband, Tom's father, who died of cancer a few years ago. Nanoo explains that she has forgotten him in many ways and that, though it sounds strange, she doesn't necessarily miss him. He was always distant during his life, and Nanoo admits that there were some things she never knew about him, like if he ever had any dreams. The next morning, Tom goes for a walk in the snow, remembering his fear of people hidden in the woods when he was a child and wondering about the tiny organisms living in the stream.