Maguire is a sinister assassin who works as a crime scene photographer, and he has a very pale skin complexion due to constantly working in a darkroom. When Michael Sullivan (an enforcer for Irish mob boss John Rooney) takes it upon himself to kill Rooney's unstable son, Connor, after Connor murders Sullivan's wife, Sullivan contacts the gangster Al Capone to try and get work and also to discover the whereabouts of Connor. Unfortunately, Capone's underboss Frank Nitti refuses to help him and instead informs Rooney of their meeting. Rooney, who loves Sullivan like a son but who also fears for Connor's safety, reluctantly allows Nitti to hire Maguire to assassinate Sullivan. As Sullivan and his son drive to a roadside diner, Maguire tracks them and engages in conversation with Sullivan over a cup of coffee. However, Sullivan soon realises Maguire's hostile intentions and attempts to escape with his son. Maguire immediately attempts to kill them and murders a police officer in the process, but Sullivan manages to puncture Maguire's tire before fleeing in his car.
Later, after Sullivan tries to take revenge on Capone for ordering the hit by robbing banks containing Capone's laundered money, Sullivan is led into a trap after meeting with Rooney's accountant Alexander Rance at his hotel. Maguire, who is staying in the hotel across the street, notices Sullivan through the window and races over to Rance's hotel suite to finish the job. Rance, knowing that Maguire is on his way, intentionally stalls Sullivan until he finally enters, and a gunfight ensues. Rance is killed in the crossfire and Maguire is injured, though the assassin manages to wound Sullivan in the shoulder before he escapes a second time.
After Sullivan finally avenges his wife's murder by killing Connor, he and his son travel to their aunt's beach house in Perdition. Once they enter, Sullivan is shot by a hidden Maguire, who had been waiting for them. As Maguire is about to photograph the dying Sullivan, his son points a gun at him and threatens him with it. Maguire menacingly walks towards him, telling him to hand the gun over to him, but the son cannot bring himself to shoot. Before Maguire can reach him, the dying Sullivan pulls out his own gun and shoots Maguire in the back, killing him. Sullivan then succumbs to his own wound and dies in his son's arms.
Maguire photographing a crime scene
Maguire collecting clues on the whereabouts of his target, Michael Sullivan
Maguire trying to get Sullivan's phone number
Maguire driving in his car at night
Maguire making friendly conversation with Sullivan at a diner
Maguire shooting at Sullivan as he tries to escape
Maguire looking out of his hotel room window
Maguire engaging in a gunfight with Sullivan after he walks into a trap
Maguire about to photograph the dying Sullivan after shooting him
Maguire approaching Sullivan's son, Michael, as he points a gun at him