Rose Lorkowski (Amy Adams) is a thirty-something single mother, working full-time as a maid. Her underachieving sister Norah (Emily Blunt) lives with their father Joe (Alan Arkin), and is fired from her job as a waitress. After the school expels Rose's son Oscar for his erratic behavior, she is told she must put him on medication or send him to a private school. Unable to make enough money with her current job, Rose asks Mac (Steve Zahn), her ex-boyfriend during their high school years and her married lover, for advice. Mac recommends a crime scene cleanup job, and with his connections as a police officer, he gets Rose and Norah into the business.
At first, the sisters perform their job poorly, carelessly handling the hazardous materials by throwing them into dumpsters instead of properly disposing of them in an incinerator. Needing to operate as a more reputable service, the sisters get the necessary tools from Winston (Clifton Collins, Jr.), a one-armed storekeeper of a shop for cleanup material. The sisters name their cleanup business "Sunshine Cleaning" and start making progress as their reputation grows. They begin to find meaning in their function to "help" in some way in the aftermath of a loss or disaster, even though the job stirs up memories of their own mother's suicide. At the same time, the members of the family deal with their individual problems. Rose encounters some of her former high school classmates and is embarrassed by the state of her position in life. After an encounter with Mac's pregnant wife in a gas station, Rose realizes that Mac will never leave his marriage, and ends their relationship. Norah meets and has a relationship with Lynn, the daughter of a woman whose house they cleaned. Joe begins to sell shrimp, hoping to raise enough money to buy a pair of expensive binoculars that Oscar wants for his birthday.
One day, an insurance company calls for the services of Sunshine Cleaning, granting the sisters the potential to obtain a breakthrough reputation. Unfortunately for Rose, a baby shower is on the same day, with all her high school classmates attending. She asks Norah to clean the house alone until she can catch up. Norah's attempt to clean the house is disastrous as she accidentally burns down the house with an unattended candle. The burning of the house tarnishes the business reputation and they are forced to pay a debt of $40,000, which the sisters simply cannot afford. Sunshine Cleaning goes out of business, and Rose is forced to return to her previous job as a maid. Meanwhile, Joe's shrimp plan goes awry as all the stores and restaurants he approaches refuse to buy. Joe had not realized when he purchased the shrimp that no legitimate business would be willing to accept the health risks involved in buying food from a non-reputable source. Lynn becomes frustrated as she questions whether Norah was truly interested in her at all. At Oscar's birthday party, Norah apologizes to Rose, and despite still being mad at her, Rose forgives her. The family and Winston then celebrate Oscar's birthday.
Sometime later, Rose visits her father who says he's sold his house and explains that with the money, he has started, with Winston's help, a new cleanup business named Lorkowski Cleaning. He asks Rose to go into partnership with him and she agrees. Norah goes on a road trip to find her new self, while Rose starts working with her father at Lorkowski Cleaning. The movie ends on a positive note, with the implication that Norah has reached a positive turning point in her life and is at peace with herself and that Rose will successfully run her business while also looking after her eccentric father.

Question: Who is Mac?
Answer: Mac is Rose's ex-boyfriend and lover who is married to another woman.
Alternative answer: Mac is Rose's ex-boyfriend.