The powerful superhero John Hancock has become a joke because of his alcoholism and clumsiness. He has also become the most hated man in Los Angeles. Though he has saved many lives, he also destroyed a lot of property, costing the city millions every time he goes into action. When he saves the life of PR expert Ray Embrey from an oncoming train, the executive is thankful and believes he can restore Hancock's image as a true superhero. He brings the anti-hero home for dinner and introduces him to his son Aaron, a big fan, and to his wife, Mary. But for some mysterious reason Mary doesn't want Hancock anywhere near her or her family.
Hancock is a superhero whose ill considered behavior regularly causes damage in the millions. He changes when the person he saves helps him improve his public image.
- John Hancock (Will Smith) is a drunkard with superhuman powers, including supersonic flight, invulnerability, immortality, and super-strength. Although he uses his powers to rescue people and stop criminals, his activities inadvertently cause millions of dollars in property damage due to his constant intoxication and cynical attitude. As a result, he is routinely jeered by the public and is considered a nuisance by the LAPD. Hancock frequently ignores court subpoenas and lawsuits from the city of Los Angeles to address the property damage he has caused.
When public relations spokesperson Ray Embrey (Jason Bateman) departs from an unsuccessful meeting pitching his All-Heart logo for corporations that want to be seen as charitable, he becomes trapped on railroad tracks facing collision with an oncoming freight train. Hancock saves Ray's life at the cost of derailing the train and damaging other cars. Hancock is jeered by other drivers for causing more destruction, but Ray steps in and thanks Hancock for saving his life. Ray offers to improve Hancock's public image, and Hancock grudgingly accepts. Ray convinces Hancock to turn himself in for his outstanding subpoenas so they can show Los Angeles how much the city really needs Hancock when they miss him fighting crime and saving lives. When the crime rate does rise following his incarceration, Hancock is contacted by the Chief of Police to help stop a violent bank robbery. With a new costume from Ray, Hancock is released from jail and makes a triumphant return by rescuing a wounded police officer, and foiling the robbers lead by Red Parker (Eddie Marsan).
Hancock is applauded for handling the bank robbery and becomes popular once more, as Ray had predicted. He goes out to dinner with Ray and his wife Mary (Charlize Theron), with whom he reveals his apparent immortality and his amnesia from 80 years ago. After Hancock tucks a drunken Ray in bed, he discovers that Mary also has superhuman powers. He threatens to expose her unless she explains their origins. Mary eventually tells him that they have lived for 3,000 years with their powers, having been called gods and angels in their time. She also explains that they are the last of their kind and are meant to be paired. Mary does not tell Hancock the entire truth, and Hancock departs to tell Ray about the conversation. The exchange results in a battle between Hancock and Mary that takes them to downtown Los Angeles, causing significant destruction to the area. Ray, downtown in a business meeting, sees and recognizes his wife using abilities like Hancock's.
Hancock meets Ray and Mary back at their house. Mary explains that Hancock is technically her husband, explaining that they were built in twos, and that they are drawn to each other over time and great distances. When later intervening in a liquor store robbery, Hancock is shot and wounded. Visiting him at the Hospital, Mary explains that when a pair of immortals get close to each other physically, they begin to lose their powers. She also tells him that she and Hancock have been attacked as a couple many times throughout history, most recently being in an alley in Miami 80 years ago. His skull was fractured during the attack, causing amnesia. To save his life at the time, Mary deserted him, allowing him to recover from his injuries. After her explanation, the hospital is raided by the bank robber Red Parker and two other criminals that Hancock had encountered when imprisoned. Mary is shot trying to defend Hancock as he is able to stop the two men, but is further wounded in the process. When Red attempts to finish Hancock off, Ray comes to the rescue and stops the bank robber with a fire axe. With Mary dying, Hancock uses the last of his strength to flee from the hospital so that their parting would allow her to heal with her powers. Hancock is now living in New York City, working as a superhero there. As gratitude to Ray, Hancock paints Ray's All-Heart logo on the moon, giving worldwide advertisement to his cause.