20. In Extremis

When a highly decorated doctor named Richard Nelson ( Dennis Boutsikaris ) is poisoned with a fatal dose of radiation, Reese is forced to partner with Nelson to track down the killer after they both realize that the radiation has no antidote. Unable to save Nelson, the two poison the killer with the same toxin. Flashbacks explore how Detective Fusco became involved with HR and the development of his friendship with Detective Stills. Meanwhile, HR has an informant give the Internal Affairs Bureau the location of Stills' body so that they can send Fusco to prison in retaliation for him quitting.

Fusco confesses his past to Carter who is forced to evaluate her partnership with Fusco and how she might help him with the problems he is facing. With the help of Bear, Carter finds and moves Stills' body while Finch has Elias get the informant to recant. The Machine displays the coded blue screens with increasing frequency as it becomes apparent to Finch and Reese that the numbers are coming too late, leading to Dr. Nelson's death. As the episode ends, the Machine sends a series of alerts before beginning the process of shutting down its primary functions.