One day two men break into his house where a scuffle ensues. The men overpower Shelby. They rape and murder his wife, although Shelby shoots one of the intruders dead. In the following melee, the second man ambushes Shelby and forces his head into a mirror whereupon he sustains serious injuries to the head. The injuries are so severe that they affect Shelby's memory. He cannot build new memories from then on. That means he forgets everything that happens in his life as soon as he experiences them. The only memory he has is that of the attack. Shelby goes on a quest to avenge the murder of his wife, investigating along the way.
So he comes up with a system to compensate for his condition. Whenever he discovers information that may be helpful in his quest to find his wife's killer, he writes it down or tattoos it on his body. It is through those tattoos and notes that Shelby is able to track the long-term progress of his cause even as he forgets minor events on a daily basis. Every morning when he wakes up he checks the tattoo and the notes and knows once again that he is engaged in a quest to find someone, what that person may look like and who are his enemies and friends along the way. The Shelby who finds and kills that man is a very tattooed, one but he does achieve what he set out to do despite the memory problems.
