As children, Sammy and Terry Prescott lost their parents to a car accident. Years later, Sammy (Laura Linney), a single mother and lending officer at a bank, still lives in her childhood home in Scottsville, New York, while Terry (Mark Ruffalo) has drifted around the country, scraping by and getting in and out of trouble.
After months of no communication with his sister, Terry is desperate for money, so he comes to visit her and her son Rudy (Rory Culkin) who are excited about reuniting with him. Sammy lends him the money, which he mails back to his girlfriend. After the girlfriend attempts suicide, he decides to extend his stay with his sister, which she welcomes.
For a school writing assignment, Rudy imagines his absent father as a fantastic hero. Sammy only gives him vague descriptions of the truth while Terry lets his feelings be known about Rudy Sr.'s abandonment. Sammy rekindles a relationship with an old boyfriend, but is surprised when he proposes to her after a short time. She needs time to consider it.
At the bank, the new manager, Brian (Matthew Broderick), tries to make his mark with unusual demands about computer color schemes and daily timesheets. He is particularly tough on Sammy, requesting that she make arrangements for someone else to pick up her son from the school bus rather than leaving work. After some minor arguments, they end up having an affair, despite Brian's wife's being six months pregnant.
Terry grows close to Rudy during their time together. Yet he pushes the limits of Sammy's parental control during a late-night game of pool at a bar. She turns to her minister (Kenneth Lonergan) to counsel Terry about his outlook on life. While Terry resists his sister's advice, he stays on good terms with his nephew. Realizing her own questionable decisions, Sammy turns down her boyfriend's marriage proposal and breaks off her relationship with Brian.
After a day of fishing, Terry and Rudy decide to visit Rudy Sr. in the town of Auburn. Confronted by his past, Rudy Sr. (Josh Lucas) is incensed, leading Terry to assault him and get arrested.
Sammy brings her brother and son home and asks Terry to move out, which he does the next day. He plans to go back to Alaska and scoffs at Sammy's suggestion to remain in town and get his life back on track. While at first it appears the separation will be another heartache, they reconcile before Terry leaves, coming to terms with their respective lifestyles.

Question: Who does Brian have an affair with?
Answer: Sammy
Alternative answer: Sammy