Reese and Finch question the machine's reliability when they are asked to investigate a teenager who was killed years earlier; Finch remembers the origin of the machine.

The Machine gives Reese and Finch the number of a down-and-out man who has been pushed too far, and they're forced to turn to former Person of Interest Zoe Morgan for help.

As the clock ticks down, Reese winds up trapped with his POI in the most heavily surveilled part of the city, forced to rely on the help of Finch, Carter and Fusco to fend off the FBI, corrupt cops and a slew of old foes.

With Finch missing – kidnapped by his hacker nemesis, Root, Reese enlists Detectives Carter and Fusco to get his friend back, Meanwhile, The Machine never sleeps, disseminating a new number to Reese.

Finch goes under cover to protect a youth who reminds him of himself; Carter goes on a dangerous mission for the FBI.

Finch and Reese team up with unexpected allies as they race to fix the machine; as Detective Carter tries to apprehend Cal Beecher's murderer, she finds herself up against HR -- a criminal organization.

Flashbacks to Finch’s adolescence provide insight into his creation of The Machine. Also, the team receives the number of an already dying man, and Reese leaves the POI team behind in the aftermath of the war with the crime organization, HR.

Multiple enemies converge on the POI team as they protect a dying man whose number has come up. They struggle to keep their enemies at bay but without Reese, who has left them and their mission behind following the tragic war with HR, being down one man could have catastrophic repercussions for the team.

Reese poses as a Secret Service agent to get close to the latest POI, a U.S. congressman who may hold the key to derailing Decima’s plans to bring a second Machine online.

Forced to take on new identities provided by Root, the team members try to adapt to their new lives. However, some find it hard to ignore The Machine's numbers, which puts them all at risk of being detected by Samaritan.

Reese volunteers as an NYPD police academy instructor in order to keep an eye on a talented young officer whose suspicious actions make him question her motives. Meanwhile, Martine – armed with information from Samaritan – hunts a member of the team.

Reese must save Elias from the Brotherhood. He can't get any help from Root or Shaw since Shaw's cover is blown and Root is escorting her to underground.