The film begins in 1981 in San Francisco, California. Linda and Chris Gardner live in a small apartment with their son, Christopher. Chris has invested the family's life savings in a franchise selling portable bone density scanners. These scanners provide slightly denser pictures than X-rays, but most of the doctors Chris visits find that they are too high-priced. Linda works in a dead-end job in a local hotel laundry. The tension between them mounts as unpaid rent and bills continue to accumulate. Chris often parks his car in disallowed areas so he can make scheduled appointments on time, and after parking tickets remain unpaid, their car is impounded. After missing a shift at her job, Linda finally leaves without their son Christopher, returns briefly, then departs for New York City, where a better job awaits her, leaving behind the boy at his father's request.

Chris accepts an unpaid internship at brokerage firm Dean Witter Reynolds that promises employment to only one trainee at its conclusion. His lack of salary, and his lack of scanners to try to sell, leaves him riddled with debt, and he and his son eventually become homeless. After spending several nights riding buses and sleeping in subway restrooms, saddled with their meager belongings, they begin lining up at the Glide Memorial Church on a daily basis in an effort to secure accommodations for the night. Sometimes they succeed, other times they literally are left out in the cold. As he struggles to provide a semblance of a family life for his son under the most dire of circumstances, Chris becomes more determined to complete the intern program and become the sole trainee the firm will hire.

In the end, Chris gets the job and later starts his own brokerage firm in 1987. Then in 2006 he sells a minority of it for a multi-million dollar deal.