Following a deadly chemical attack on the Washington, D.C., Metro, Red pins the blame on Frederick Barnes, a former defense research scientist who’s been selling chemical weapons to the highest bidder since quitting his government job five years ago. Now that he’s not just making weapons, but using them, too, Red says Barnes is the most dangerous man in the world. After the 37 subway victims are found to have died from a rare vascular disorder called Kurz disease, traces of strontium-90 found on the briefcase used to deliver the bomb sends Red in search of the man who supplied Barnes with the radioactive isotope. While questioning Barnes’ former research assistant, Anne Forrester, Liz notices that her ten-year-old son Ethan also has Kurz. When pressed, she admits he’s Barnes’ son, something she’s never told her husband. So after Anne reveals that Barnes couldn’t get anyone interested in Kurz research, Liz concludes that the attacks are meant to call attention to finding a cure.
When Red arrives in Havana to buy strontium-90, the supplier, Manuel Soto, says he’s all out. Red asks to be put in touch with his previous buyer in hopes of striking a deal, so Soto contacts Barnes, allowing Red’s team to trace the call to a courthouse in Virginia. Spotting Barnes after he detonates another deadly device in a jury room, Liz gives chase. Following a tense standoff during which Liz refuses to endanger a police officer he’s holding hostage, Barnes escapes. However, when she admits she didn’t shoot so as to avoid risking the hostage’s life, Liz is suspended.
After learning of a victim who survived the courthouse attack, Barnes poses as a nurse and gains access to the hospital to get a bone marrow sample he can use to try and cure Ethan. Certain that his next stop is the Forrester home, Liz calls to warn Anne to get Ethan to safety just as Barnes shows up looking for them. When she and Meera arrive, this time Liz doesn’t hesitate to shoot even when Barnes uses Ethan as a human shield. Finally, as Liz and Tom put their marriage back together now that he’s been exonerated in the murder of a Russian defector, Red concludes a series of financial transactions to buy the house where he raised his family, only to turn around and destroy it.