Jerry Landers (Bob Denver), an assistant manager at a Food World grocery store in Burbank, California, receives a letter at his home after work summoning him to a meeting with God the following day. Jerry and his wife, Bobbie (Terri Garr), dismiss it as a practical joke and he tears the note up and throws it away. However, the next day, the note turns up in a head of lettuce at the store and Jerry feels compelled to honor the request.
Jerry goes to the appointment meeting place in downtown Los Angeles, California, and finds an office with nothing but a chair and an intercom and he is greeted by a voice on the speaker. The voice attempts to convince the dubious Jerry that he really is God by demonstrating that Jerry is on the 27th floor of a building that actually has only seventeen floors. After Jerry leaves the building completely spooked, the voice begins talking to him through Jerry's supposedly broken car radio. God informs Jerry that he wants him to spread the word that God exists.
At home, Jerry tells his wife all about the meeting and encounter. But naturally Bobbie does not believe Jerry and suggests that they go on vacation and visit his brother-in-law, who is a therapist, but Jerry insists he is not crazy, which fuels her suspicions about Jerry and his mental health.
The following day, God speaks to Jerry in the shower and then appears to him in the guise of an elderly man (George Burns). When Jerry asks him how He can permit so much suffering in the world, God explains that the world is based on luck and free will. God tells Jerry that it is up to him to inform the world to heed his message, treat one another better, and look after the earth's natural resources. God urges him to go to the press.
Jerry visits Briggs (George Furth), a Los Angeles Times reporter, that God asked him to be His messenger, but the journalist is skeptical. Later, God appears to Jerry at the grocery store and asks him to go for a drive. Jerry asks for a sign and God makes it rain inside the car. When Jerry is pulled over by a policeman to inquire about the water leaking from the moving car, Jerry lies to the motorcycle cop by claiming that he left the car windows down while driving through a car wash. After the policeman lets Jerry go with a warning, God is somewhat disappointed that Jerry lied to the cop to avoid getting in trouble and insists that Jerry spread his word with truth and no more lies. Jerry returns to the newspaper office (still soaking wet) and again tries to convince Briggs.
The next day, the newspaper runs a story that includes Jerry among tales of various religious fanatics and Bobbie is mortified. Jerry explains that he has now seen God and needs her to believe him. She allows that Jerry believes he spoke to God, and they attempt to explain the situation to their children, Adam and Becky, but the boy and girl hope their father is just going through a phase and also think that he may be crazy. Later, a television news program picks up the story and the media surrounds the Landers' Tarzana, California, home. A Food World executive warns Jerry not to make any more claims that he has spoken to God if he wants to keep his job.
In another meeting, God tells Jerry that there are no guarantees in life, and his job is a small price to pay for saving the world. When Jerry picks up Adam at school, his son is embarrassed to be seen with him, and Becky refuses to speak to him as well. At home, Bobbie pleads with Jerry not to jeopardize his career or their lives as their neighbors are now avoiding her.
Later, Jerry appears as a guest on the Dinah Shore television show, on which a Los Angeles Police Department sketch artist produces a drawing of God based on Jerry's description. After the show airs, people begin to recognize Jerry where ever he goes. Bobbie is extremely upset when a crowd of religious zealots gathers outside the Landers' house.
A committee of religious leaders of various faiths calls Jerry before them to verify his story. The committee sequesters Jerry in a hotel with fifty questions written in Aramaic, which he is to answer with God's help. God appears as a room service waiter and dictates the answers to Jerry and instructs him to take them directly to Reverend Willie Williams (Paul Sorvino), an evangelical preacher, whom God calls a fraud.
Jerry attends Reverend Williams's revival meeting at the Shrine Auditorium and tells Williams's followers what God said which leads to a riot at the auditorium and Jerry getting thrown out by the hostile bounders.
Reverend Williams then sues Jerry for slander. In court, Jerry acts as his own attorney and calls God to the witness stand. The judge threatens Jerry with contempt of court, but God enters the courtroom and testifies. Everyone in court can see and hear Him and He performs a card trick and makes himself disappear to prove his legitimacy; however, when the audio recording made by the court clerk is played back, God's voice cannot be heard and His testimony disappears from the stenographer's transcription. The judge dismisses the slander charge against Jerry, but rules there is no evidence that God appeared despite their mutual experience of the phenomena. Never the less, Jerry and Bobbie reconcile as she now believes him and of his encounters with God.
As the film comes to an end, Jerry loses his job at Food World, collects his final check, and says goodbye to his co-workers. As he drives away, Jerry hears a payphone ringing in a park and stops to answer it. God materializes in the adjacent phone booth. Jerry asks if they failed to spread his message due to him loosing his job and all of his friends and neighbors still ostracizing him. God assures Jerry that they succeeded in getting His word out and now they just have to see what happens. With Jerry's work complete, God bids him goodbye and fades away.