Everything I Never Told You examines the Lees, a middle class mixed-racial family in suburban 1970s Ohio. After middle child Lydia disappears and is discovered dead at the bottom of a lake, the Lees will have to revisit painful memories that have contributed to extremely tense and fragile family dynamics. Through shifting perspectives and shifts in time, Everything I Never Told You explores the danger in silence and repression and the repercussions of never voicing how you really feel.

One spring day in 1977, Marilyn and James Lee sit down to breakfast with their son, Nath, and youngest daughter, Hannah. Gradually, the Lees realize that their middle daughter, Lydia has not come down for breakfast and, in fact, nobody knows where she is. The police are informed once the children go off to school, and eventually it is discovered that Lydia drowned in the lake nearby the Lee's house. As the novel opens, the reader sees that Lydia is a mystery to her family just as much as she is a mystery to the reader. Certain things do not add up: Why would Lydia row herself out to the middle of the lake if she cannot swim? Was it Lydia that Hannah heard leaving the house at 2 a.m.?

To help answer this question, the narrator takes the reader back to the 1960s, when Marilyn and James met at Harvard when he was a young graduate student teaching a course that Marilyn took. Their love affair seemed to erase their insecurities--his for being Chinese American, hers for being doomed to end up a homemaker like her mother--and Marilyn becomes pregnant with Nath, and she leaves Harvard and her medical school aspirations to marry James and start a family together.

At Lydia’s funeral, Nath expresses his suspicions that their neighbor Jack was involved in Lydia’s death. Their argument nearly comes to blows, but it is broken up by James. James drives to his office at the university where he teaches, and his young, Chinese American teaching assistant, Louisa, invites him back to her place for something to eat. They begin to carry on an affair. Marilyn promises herself she will discover the truth about Lydia’s death. She spends time searching Lydia’s room for answers and is only left with questions as she realizes she might not have known her daughter after all.

The narrative switches back to ten years prior to the present day when Marilyn begins to feel restless after her mother dies. James takes Nath to the community pool and witnesses his son being the victim of racist harassment much like he has felt throughout his life. Marilyn realizes she still wants to be a doctor, and she plots to abandon her family and move to Toledo so she can finish her pre-med requirements. She leaves without explanation, tearing up a note to James that he pieces together.

In present day, Hannah wonders about her sister’s life. Marilyn and James argue over the cause of Lydia’s death, if it was a murder or suicide. James and Louisa’s affair intensifies. The narrative then returns to the summer that Marilyn disappeared and talks of how the children were left to their own devices by a distraught James. Nath takes an interest in space travel and astronomy, which he eventually intends to pursue in college. Marilyn’s studies are interrupted when she discovers she is pregnant with Hannah. She is forced to come home, and Lydia promises herself she will do whatever her parents ask. Marilyn begins to shape Lydia’s studies so she will become a doctor. This intense pressure on Lydia continues to present day. The family begins to orient themselves around Lydia and her future.

In 1976, Lydia is failing physics, a secret she has kept from her mother. Lydia has intercepted a letter to Nath telling him that he has been accepted to Harvard. Eventually Nath is given another letter, and his celebratory moment with his family is interrupted when Lydia tells everyone of her struggles with physics. This causes tension between Lydia and Nath, who is finally getting tired of having everyone’s attention focus on Lydia, and Marilyn is furious. When school resumes, Lydia decides to start a friendship with Jack, the Lee’s neighbor and someone who Nath detests. Previously Hannah had discovered that Jack is in love with Nath.

After Lydia dies, Nath suspects his father is having an affair. Marilyn and James have a fight after a police officer calls to tell James they are ruling Lydia’s death a suicide. They both reveal their frustrations with their marriage. James’ frustrations are mainly because he feels self-conscious about his heritage and being “different,” and he also suspects Marilyn views their marriage as a mistake. James flees to Louisa’s apartment. Nath tells Marilyn of his suspicions that James is having an affair, and Marilyn tracks him down at Louisa’s apartment but does not confront him.

The plot then weaves back to earlier in the year when Lydia and Jack are becoming friends. He is helping her learn how to drive, and he also listens to her problems. Jack also asks endless questions about Nath, but Lydia does not mind because it keeps him talking. On the night before Lydia’s 16th birthday, her father gives her a locket, but Lydia feels like the present is ruined by James’ hints that it is trendy. The next day, Lydia begins to suspect Louisa and James are having an affair, and she also fails her driver’s permit test.

In July 1977, James is hurrying home from Louisa’s apartment. He and Marilyn have an angry confrontation that ends in him storming off and telling her that they should pretend like their marriage and children never happened. James drives to Toledo but turns around when he realizes that years ago when Marilyn left, she still came back and stayed.

Returning once more to May 1977, the narrative examines the final days in Lydia’s life when she feels like Nath is blowing her off and minimizing her feelings. Lydia attempts to initiate a romantic encounter with Jack, but Jack rejects her and tells her about how he pines for Nath. Lydia decides to go out in a boat on the lake and step into the water, believing she can swim to shore. She drowns.

In July 1977 James returns home and he and Marilyn embrace and renew their love for one another. Hannah has begun to bond with her parents more. She follows Nath as he storms out to the lake after Jack. Nath tries to initiate a fight with Jack and punches him, but Hannah breaks it up before it can go any further. Hannah accidentally pushes Nath in the water, and Nath feels a connection to how Lydia must have felt in her final moments alive. Seeing Hannah’s face makes Nath feel like he has something to hold on to, and he swims ashore.