When a person of interest is poisoned, Finch and Reese only have 24 hours to identify the toxin and find the person responsible; Detective Fusco's past catches up to him.

Finch and Reese must delve into the murky waters of international politics when they are given the number of a former government contractor who is stalking a UN diplomat. Also, an enemy attempts to gain an upper hand on Root.

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 and Finch attempt to rescue the Machine’s source code before it deteriorates beyond repair. Also, Root fights for her life against an onslaught of Samaritan’s agents and Fusco is faced with hard questions following the deaths of Dominic and Elias.

Reese and Finch become trapped in a hospital that becomes ground zero for a deadly viral outbreak. Also, Samaritan’s newest recruit has second thoughts and Shaw continues to struggle with reality.