Young Tracy Richards (Louanne) and her neighbor, Shingo (John Louie), shoot basketballs until Tracy's divorced father, Don Richards (David Birney), arrives in his red Jeep to take her on an outing. Don's ex-wife and Tracy's mother Paula (Suzanne Pleshette), tells Don to have Tracy back at a regular time.
As father and daughter eat Chinese food, Don complains about the plot from a science-fiction movie they saw, but Tracy talks about the importance of believing in supernatural things. As Don leaves to make a phone call, Tracy opens a fortune cookie that reads: " Meet me in the lounge. - God." A second fortune cookie says, "I mean you, Tracy. - God."
In the lounge, a man's voice beckons her through a red lacquer door, introduces himself as God, and asks for help. At first, Tracy thinks her father is playing tricks, but when other people cannot hear God's voice, she is convinced. God also says he was impressed when she talked about the importance of believing things that cannot be seen. Then, the voice goes silent.
On the ride home, Tracy says she had a daydream although vague on the details, but Don, an advertising art director, assures her that it happens to people all the time. He then leaves her in the parked car while retrieving work from his office. This time, God appears in the backseat as an elderly man (George Burns) wearing a gray jacket, black-rimmed glasses, and a red shirt. When Tracy said she expected a deity to wear a crown and a flowing robe, God tells her she has confused him with the actor Charlton Heston. God complains that people do not believe in him anymore, so Tracy suggests that he needs an advertising slogan to spread his message. He asks her to create one that children will understand. Later, Don gives Tracy some pointers on ways to devise a slogan.
Back at home, Tracy asks Rosa (Alma Beltran), the housekeeper, if she talks to God. Rosa admits to praying everyday, but says that Tracy's imagination is running wild when Tracy confesses to her experience with God. Later, Tracy tells Shingo about her adventure with God, but he does not believe her either. When Tracy angrily throws a basketball at the hoop and makes a perfect shot, Shingo wonders if she is telling the truth.
At school, Tracy's teacher, Miss Hudson, accuses her of being inattentive in class and not turning in her homework, but Tracy is unable to explain her lack of interest. Back at home, Tracy studies television commercials obsessively, trying to figure out an appropriate slogan for God. As she watches The Johnny Carson Show, God appears on the television, and then, suddenly materializes in Tracy's living room. After listening to Tracy's material, God gives the girl another week to create a winning slogan. He encourages her to bounce ideas off of Shingo, with whom she and he eventually come up with "Think God."
At a local McDonalds the next day, Tracy presents the slogan to God, he tells her the next step is to spread the word. Tracy is worried that the extra responsibility will make her fall farther behind in her schoolwork, but God is persuasive. Tracy rallies her schoolmates, who post signs and leave chalk messages all over town. However, Paula Richards confronts her daughter after receiving a letter from Miss Hudson. When Tracy explains that God has visited her, Paula does not believe her. Tracy is pressured to catch up on her schoolwork, while Shingo enlists his cousins to help spread the word.
Later, Tracy complains to God that spreading his slogan has gotten her in trouble with both her parents and friends, and he soothes her by finishing her math homework.
Another time later, Don and Paula question their daughter, but Tracy sticks to her story about God. Don wants Tracy to see a psychiatrist or a minister, but Paula wants to handle Tracy's situation by herself without outside interference. The parents decide to watch Tracy's behavior for a few more days.
Back at school, the principal, Mr. Benson, arranges a meeting with the Richards because Tracy refuses to stop her God campaign. When Paula tells her daughter that she will be expelled from school if she continues, Tracy says she will stop on the condition that God agrees to the plan. When the school psychologist learns that Tracy's relationship with God started in a Chinese restaurant with a fortune cookie, the principal suspends Tracy and the psychologist tells the parents their daughter needs professional help.
Meanwhile, Tracy's school friends picket in front of the school for Tracy's return. Soon, Don and Paula take Tracy to see Dr. Jerome Newell (Anthony Holland) who examines Tracy and puts her through a series of psychosis tests. After the tests are done, Dr. Newell tells Don and Paula that he theorizes (wrongly) that Tracy has created a "fantasy father figure" as a substitute for her real father, whom she sees on a limited basis since her parents' divorce. Dr. Newell concludes that there is nothing medically wrong with Tracy, but she is suffering from a psychosis. He suggests that she enter a mental treatment facility in Santa Barbara, California. Before Tracy is taken away to be admitted, she searches for God in several churches and even a synagogue, but cannot find him.
Tracy decides to run away, and Shingo helps her hide at his grandparents' house. Later, Tracy runs away to Union Train Station after Shingo warns her that he had to reveal her whereabouts when Don confronted and threatened him. There, God finally appears and convinces Tracy that running away is not the answer, and drives her home in a motorcycle sidecar. Two policemen see the motorcycle (with no one riding it) and chase after it, but God manages to loose the policemen. When Tracy's parents find their daughter in bed, Tracy explains that God brought her home.
Meanwhile, the news reports that Tracy's slogan has spread internationally. In court, a judge asks that a panel of psychiatrists convene to determine Tracy's mental health. Based on their findings, the judge says he will rule on whether or not Tracy goes back to school. As the doctors' panel rules that Tracy can start treatment, God arrives and introduces himself as Dr. Stevens, a psychiatrist. Demonstrating his supernatural power, God makes a chandelier disappear and reappear, and turns day into night. He then turns to leave and vanishes; only the door opens and closes by itself. The panel is left speechless, so the Richards take their daughter home.
Tracy and her family celebrate at the Chinese restaurant. Tracy receives a message from the fortune cookies and goes to the lounge. There, God thanks her and her friends for their wonderful campaign. Although Tracy is sad she may never see God again, he promises to call on her if he needs help with a big problem.