Dennis Zalewski is a prison guard at Shawshank State Prison. He is expecting his first child with his wife, and is disgusted at the way the prison is run and how the other corrections officers treat the prisoner. After the suicide of Warden Dale Lacy, Dennis finds a mysterious prisoner in a cage built into a water cistern beneath a disused section of the prison.
Dennis Zalewski works at Shawshank State Prison as a corrections officer. He is not fond of the job, remarking that if there was a Walmart within twenty miles then he would be working there instead. Dennis' wife is pregnant with their first child, and he is reluctant to do anything that would endanger his job. However, he is disturbed about the way the rest of the prison staff treat the inmates at Shawshank.
After Warden Dale Lacy's suicide, Dennis is on duty when the new warden, Theresa Porter, arrives. Porter is looking for more space to house inmates; the prison is now owned by Northeast Correctional and they are looking to turn a profit with more prisoners. Dennis informs her that the gym is already being used to house prisoners, and there is no more space – apart from the disused Block F which has been closed since the 1980s due to a fire that killed a lot of prisoners. Porter sends Dennis and Boyd to Block F to count the beds.
Entering Block F, Dennis finds signs that someone has been in the disused block recently and follows a trail of footprints in the dust. He finds a hatch to a water cistern, and inside there is a cage with a mysterious prisoner inside. The prisoner is completely unknown to the prison – apparently Warden Lacy was keeping him prisoner for his own reasons. The prisoner says only one thing: a name, "Henry Matthew Deaver."
Dennis does not recognize the name, but Boyd does and knows that the prisoner is not Henry Deaver. Henry Deaver caused a stir in 1991 when he went missing for eleven days, during which his adopted father, Reverend Matthew Deaver, was found injured after a fall. Deaver would die three days later. Henry is now a lawyer specializing in Death Row cases. Not wanting a scandal to engulf the prison regarding an off-the-books prisoner, Warden Porter orders no one to talk about his presence and no effort is made to contact Henry. Dennis takes matters into his own hands and calls Henry anonymously, telling him that one of his clients is requesting his services.
The prison stymies all efforts by Henry to contact his "client," a situation that is not helped since he does not the prisoner's name, nor can he pick out one of his clients in the prison's roster. As Henry leaves the prison, Dennis is on duty ordering prisoners around. Henry recognizes his voice from the telephone call.
On duty in the monitor room, Dennis watches as the Kid seemingly escapes his cell and starts leaving dead bodies in his wake. Dennis raises the alarm and arms himself, heading to the prison floor. Nothing is amiss, and Dennis almost shoots Boyd. The Kid, meanwhile, is still in his cell.
While visiting The Mellow Tiger bar with some of the other guards, Dennis is slipped a note to contact Henry. Driving to outside Henry's mother's house, Dennis calls him, and they meet to talk about the prisoner. Dennis is sure that Henry will not get access to the prisoner since the prison will not admit he exists. They formulate a plan to prove his existence and get Henry within talking distance. The local church runs an outreach program to help inmates with their religious observations and Henry joins their weekly trip to the prison. While outside, Dennis arranges for the prisoner to be outside so that Henry can see him. Dennis begins acting as Henry's conduit to the prisoner, reporting back everything that the prisoner does and says.
Eventually, the existence of the prisoner is made public by Molly Strand on the local television show Local Color. The resulting scandal forces the prison to acknowledge the prisoner's existence. Dennis hopes that this would be the first step to taking down the entire system at Shawshank, but Henry insists that the focus needs to be on his client and their needs. When personal matters lead to Henry considering to withdraw from the case and leave Castle Rock, he calls Dennis to tell him that he is advising the Kid to take the settlement offered by Northeast Correctional. Dennis is upset at this turn of events, and finally snaps. While on duty, he goes on a shooting spree, killing several corrections officers before storming into Warden Porter's office where Henry is waiting. Dennis tells him he wants to testify shortly before being killed by a riot squad.
Alternate Castle Rock timeline
In an alternate timeline, Dennis Zalewski is a member of the Castle Rock Police. He interviews Henry Deaver when he finds a young black child in the basement of his father's house. Later, when the child is released into Henry and Molly Strand's care, Officer Zalewski is assigned to watch over the house. During the drive to Molly's house, she takes off into the woods with the boy and Henry. Zalewski follows in pursuit and tries to stop them, but accidentally shoots and kills Molly.