In 1989, Joy Mangano is a divorced mother of two, working as a booking clerk for Eastern Airlines. She lives with her two young children, her mother, Terri, her grandmother, Mimi, and her ex-husband, Tony in Quogue, New York. Her parents are divorced, and her mother and father fight whenever her father shows up at her home. Joy's older half-sister, Peggy, is an overachiever who constantly humiliates Joy in front of her children. Peggy and Joy's father Rudy are very close. Terri spends all day lying in bed watching soap operas as a means of escape from her life, leaving Joy to run the household. Only Joy's grandmother and her best friend Jackie encourage her to pursue her inventing ambitions and become a strong successful woman.
After divorcing his third wife, Joy's father starts dating Trudy, a wealthy Italian widow with some business experience. While on Trudy's boat, Joy drops a glass of red wine, attempts to mop up the mess, and cuts her hands on the broken glass while wringing the mop. Joy returns home and creates blueprints for a self-wringing mop. She builds a prototype with help from the employees at her father's shop. She then convinces Trudy to invest in the product. They make a deal with a company in California to manufacture the mop's parts at a low price. In order to avoid a potential lawsuit, Joy also pays $50,000 in royalties to a man in Hong Kong who supposedly has created a similar product. When the company repeatedly bills Joy for faulty parts they create, Joy refuses to pay the fees and tells her father, Trudy, and Peggy not to pay them.
Joy is advised by Trudy to take out a second mortgage on her home, in order to pay her costs. Joy needs a quick, easy way to advertise her product, and is able to meet with QVC executive Neil Walker. Neil is impressed and shows Joy his infomercials, where celebrities sell entrepreneur's products through a telethon system. Neil tells Joy to manufacture 50,000 mops. The first infomercial fails, but when she goes on QVC, Joy and her product become an overnight success. Things look up for the family, with the mop earning thousands of dollars on QVC, and Terri falls for Toussaint, a Haitian plumber Joy hired earlier in the film to fix up a leak in Terri's bedroom.
Joy's grandmother dies suddenly. Rudy and Trudy send Peggy to California to conduct Joy's company business. Afterwards Peggy tells Joy that she paid excessively raised production fees. Joy is angry and travels to California to meet with the manufacturer, who refuses to pay her back. Joy also discovers that the manufacturer is about to fraudulently patent her design. Her lawyer reveals that there is nothing they can do to prevent this, and Joy is forced to file for bankruptcy. Joy discovers that the manufacturers have been defrauding her the entire time she has dealt with them. She confronts the owner, and forces him to pay her back.
Several years later, Joy is wealthy and runs a successful business. She continues to take care of her father, even though he and Peggy had unsuccessfully sued her for ownership of the company. Terri is the only family member who does not live off Joy, finally finding stability through her relationship with Toussaint . Jackie and Tony remain Joy's most valued advisers, and the film ends with her helping a young mother develop a new invention.