The movie opens up with Roman J. Israel Esq, (Denzel Washington) an attorney, typing up a legal brief where he's stating that he's suing himself for violating his beliefs and he should be disbarred for what he's done. We first actually see Roman working at his desk in a legal office and the phone rings. His secretary tells him that his law partner has had a heart attack, so he needs to go to court and work all the cases. She specifically tells him to just ask for continuances due to the health emergency.
Roman, however, goes to court and starts lobbying for different and lower charges from prosecution and then proceeds to get into an argument with a judge over a small matter. The judge finally holds him in contempt of court that comes with a $5,000 fine. Upon returning to his law office, he's met by his law partner's daughter as well as an attorney named George Pierce (Colin Farrell) who inform him that the law firm is on a bad financial footing. They tell Roman he's going to be able to finish up some of the current cases but that the future of the office doesn't look good.
From pictures in his home and from the law office it's clear that Roman was very active in the civil rights era of the 60's and his law firm was very instrumental during that time as well as afterward. Roman is invited by Maya to speak in front of a group of young protesters to give them legal advice on what to do if they're arrested and just general legal advice for protesters. While giving the speech Roman notices that some of the women are standing and the men are sitting, and he asks why that is? The women tell him that's sexist and gendered and he insists that it's polite and that chivalry should still be a part of social justice. He ends up getting into a nasty verbal altercation with the woman and walks out. Maya chases him out onto the street where they find a homeless man laying on the street. They try to see if he's breathing and the cops pull up. The cops tell Roman to get away from the body and Roman slips his business card into the man's pocket so that if the man is dead the cops have to call him and he "doesn't end up as just some body. He's a person." Maya is impressed and tells Roman "who does something like that?"
George Pierce ends up offering Roman a job after finding out that Roman has lots of people who want his help as he handed his card out to thousands of people over the years. It also forces George to rethink his firm a bit as it's clear they're a huge law firm with tons of money coming in. Because of Roman, he decides he wants to do more pro-bono work. One case Roman is looking into sees him interviewing a young man accused of being part of a killing. The young man tells Roman he knows who killed the man and told Roman. It turns out there's a $100,000 reward for information leading to that killing by a rich family member of the deceased. Roman puts a call into the man offering the reward from a pay phone and tells him he can tell him who did it but will require cash. They agree on the deal, even though as an attorney it's against the law for Roman to give that information out and claim a reward. They settle on a cash drop in a back alley garbage can.
Roman gets the money home, and it becomes clear from his discussions with Maya and after the blowup at the protest lecture that he's "tired of doing work for the ungrateful" and not being compensated as much as many of his fellow lawyers. He takes his new found money to Santa Monica and eats fancy donuts on the beach, buys new shorts, and a couple of new suits. He looks content with his new outlook on things. He also decides to rent an expensive high rise apartment near the beach.
Roman shows back up to work at George's law firm in his new suit and hair slicked back and George tells Roman that he's going to head up their new Pro Bono work, to which Roman replies "This isn't gonna effect my salary is it?" The first case they take on they go to the prison to talk to a client who is charged with murder. Coincidentally it's the killer that Roman gave up for the $100,000. The killer knows this and has the young man who told Roman about him killed as well. So Roman knows he's in trouble and the killer tells him he knows all about what Roman did and that he's going to take care of Roman as well. Roman runs back home and starts to pack and plans to get out of the area. He rents a U haul truck and packs it with what he needs, including boxes of legal briefs for a big thing that he's been working on for decades to change the justice system and make it fairer. It's something he's been telling George about as well, and Roman carries those papers with him every day in his big brief case. Roman drives out to the desert where he thinks people are trying to kill him are after him, but it turns out their just kids speeding by.
Roman gets a call from Maya in the desert and realizes he has to come back and has to be someone who fights for social justice, not just some high paid attorney. So he returns to the city. He talks with George for a minute about what he wants to do and then starts to walk down the street. George sees that a man is following Roman, so George starts to follow them both. The man walks up behind Roman, and we hear a gun shot. Next, we see Roman's briefcase laying on the ground. It's assumed he's been shot and killed, although we don't actually see his body.
The last scene shows George walking into a courthouse with all of Roman's legal briefs on trying to change the justice system, and the court filing person begins to file them.