Arthur Poppington, a regular man who adopts a superhero persona, known as "Defendor", combs the city streets at night, in search of his archenemy, Captain Industry.
A crooked cop, a mob boss and the young girl they abuse are the denizens of a city's criminal underworld. It's a world that ordinary Arthur Poppington doesn't understand and doesn't belong in, but is committed to fighting when he changes into a vigilante super-hero of his own making, Defendor. With no power other than courage Defendor takes to the streets to protect the city's innocents.
On court order, Arthur Poppington is under the psychiatric evaluation of Dr. Ellen Park to determine if he is mentally fit to stand trial for assaulting a dry cleaner/tailor shop owner. That evaluation is more due to his actions under the guise of an alter-ego. During the day, Arthur works as a public works flag person under Paul Carter, his friend and unspoken protector who believes he is mentally slow and who he has not told that he is living in the work's yard workshop. At night also unknown to Paul, Arthur, complete with face-painted eye mask, helmet, and masking tape "D" taped to his chest, transforms into super hero Defendor, wielding a billy club, slingshot and other more unique rudimentary yet often effective weapons, fighting for what he believes is morally right in his slightly warped world. As Arthur answers Dr. Park's questions, his story is presented in flashback. That story includes his childhood under the guardianship of his maternal grandfather - with a junkie mother who abandoned him and he not knowing his father - which shaped many of his beliefs. Arthur is well known among the local thugs who believe he is a "retard", and the police department, Captain Roger Fairbanks who believes he is basically harmless despite not welcoming Defendor's vigilante justice tactics. Core among Defendor's beliefs is that some unknown person named Captain Industry, a drug dealer, is the root of all evil. Kat Debrofkowitz, a junkie hooker with the street name Angel who Arthur befriends through an encounter with a crooked undercover cop named Chuck Dooney, leads Arthur to believe that Captain Industry is a Serbian mobster named Radovan Kristic. As Defendor believes he has to rid the world of Captain Industry, he may not be aware that he is also on Kristic's radar, and as such may be in danger because of his Defendor alter ego.
- During the day, Arthur Poppington (Woody Harrelson) is a normal man (even if slightly naive to the workings of the world) working in the construction industry. During the night time, he transforms into his alter-ego, Defendor, protecting the streets from thugs. The city has a problem with thugs and nobody is prepared to stand up and fight for themselves. The police are slow to catch the wrongdoers and Arthur has a huge willpower to do what he can to help the people. He helps Kat (Kat Dennings) one night from Dooney - when she really didn't need any help - she was a hooker. Dooney (Elias Koteas) is a corrupt cop in the police force and he is in cahoots with a Serbian mob boss. Arthur has been searching for Captain industry all his life as he had killed his mother. Kat and Arthur become friends and Defendor goes after Captain Industry knowing that there is a very high chance of dying or getting arrested.