On the subway, a young woman makes conversation with a middle-aged commuter; we'll call him Frederick Barnes. Uncomfortable, Barnes exits the train at the next stop, leaving his briefcase behind. The woman runs after him, hoping to return the briefcase, but the doors have closed. A puff of gas issues from the briefcase. The girl doubles over and within seconds, all the veins on her face are visible, thrombotic and pulsating. Soon, all the inhabitants of the train are passed out, their faces etched with the blue cobwebs of their blood vessels.
At home, hoping to erase the dregs of their home invasion and everything that came after, Liz tells Tom it's time to remodel the house. Ressler calls and tells Liz to turn on the TV: thirty-seven people are dead after a biological attack on the subway. Back at the post office, Red calls into the tip line and asks for Liz, and claiming he can identify the person behind the attack: Frederick Barnes, a research scientist and savant. Barnes used to work for the government, but a few years ago, he went rogue. So why has a designer of deadly biological agents gone into business for himself?
Liz and Ressler check in with the CDC's Dr. Nina Buckner at D.C.'s General Hospital. The subway gas attack victims were killed by an extremely rare vascular disorder that causes the blood vessels to harden, depriving the body of oxygen. Usually it takes 10 years for the disease to run its course; in the case of those killed, it took two minutes - and there've been 600 cases in the last few months. Liz knows someone is weaponizing the disease - but according to Dr. Buckner, the killer would need a source for strontium-90 - a waste by-product of nuclear reactors that is highly regulated by the government.
Red knows all about strontium-90 and identifies Barnes' most likely source, located in Cuba. He asks Liz to come along, but she's more interested in tracking down Barnes' former research partner, Anne Forrester. Red throws down a gauntlet: if Liz is no longer interested in working with him, there's no fun, so he'll quit the FBI. Depressed, Red gets into his car, bound for the airport. Luckily, Luli has good news: the house Red wants to buy is within reach.
Liz and Ressler meet with Anne Forrester, Barnes' former research partner. Liz notices that Anne's son Ethan has visible blue veins: it's Kurz Disease, the same untreatable disease that killed the people on the D.C. subway. Anne admits Barnes is Ethan's birth father. Liz figures Barnes is unleashing fatal chemicals to draw attention to the rare disease in hopes of saving his son.
In Cuba, Red meets with his contact for strontium-90, Manny. During a tense negotiation, Red bugs Manny's lair, then traces Manny's first call after the meeting to get Barnes' location. Early the next morning, the FBI is waiting at a D.C. courthouse when Barnes shows up, posing as a juror. After leaving his briefcase behind, he zip ties the courtroom doors shut. Ressler and Liz are on scene, and Liz IDs Barnes in a stairwell. Ressler pulls survivor Elisa out of the courtroom just as Liz recognizes Barnes outside the courthouse. Although she has Barnes in her sights, Liz lets an armed Barnes get away.
Back at the post office, Meera posits that Barnes is conducting an experiment. Cooper pulls Liz out of the consult; as an FBI agent, she should have killed Barnes when she had the chance. Clearly, Ressler turned her in for this transgression. Meanwhile, Barnes visits survivor Elisa in the hospital and drugs her so he can run tests. Having lost the thread of the case, Liz calls Red, asking for help. He tells her to check Elisa, now unconscious. Liz and Meera deduce that Barnes has been looking for someone immune to Kurz Disease in order to synthesize an antidote for his son.
Meera and Liz call with a warning for Anne Forrester just as Barnes walks into her home. Barnes knocks out Anne, then goes for Ethan just as Liz and Meera pull up. Liz finds Barnes about to inject Ethan with his experimental serum. After her last experience with FBI protocol, Liz shoots to kill Barnes before he can inject his son with the serum. Red shows up to the crime scene to talk. Liz tries to draw a line in the sand: her personal life is off-limits. Red asks her to just say the word and he's gone. Liz says nothing, so Red leaves, ready to show up for work tomorrow.
Liz finds Tom sitting in their now empty dining room: he's gotten rid of all their furniture and made a date night out of it. Early the next morning, Dembe drives Red and Luli to the house she bought for him. When Luli asks why this house, Red allows that he raised his family there, going so far as to uncover a growth chart for his presumed daughter under some paneling. After a moment to relive his family memories, Red leaves with his companions. Moments later, the house explodes.
Following a chemical attack on the subway, Liz and the team hunt a mad scientist. Robert Sean Leonard guest stars.