In a subway station, a man in a hat and trench coat boards a busy car carrying a briefcase. A pretty redhead asks him if he bought his hat at Bergman’s. The man says yes. She bought the same hat for her father; she gave it to him at church. The man in the hat appears distraught. He disembarks at the next stop, leaving his briefcase behind. The redhead chases him with it, but the doors close. The man in the hat presses a remote control and the briefcase begins spewing smoke. Everyone on the packed subway car begins to cough; veins bulge in their face. Everyone is killed in gruesome fashion.
At her home, Elizabeth is comparing paint colors. Tom asks what she’s doing and Elizabeth says she is planning on remodeling their home. Tom picks a color called Grandma’s Pumpkins. Elizabeth explains that since someone invaded their home and planted that box, she wants to change her surroundings. She feels guilty for doubting Tom. Tom says that they just need time. They embrace. Elizabeth’s phone rings and Agent Ressler tells her to turn on the television. Elizabeth sees news footage of the massacre on the subway.
Elizabeth arrives on the scene and Ressler explains that all thirty-seven train passengers were killed. Agent Mojtabai shows Ressler and Elizabeth video footage of the man in the hat boarding the train, leaving the briefcase behind. They have identified the delivery device.
At headquarters, Agent Malik explains that she believes the attack was homegrown, not foreign terrorism. Elizabeth gets a call from Red, complaining that she hasn’t been taking his calls. Elizabeth doesn’t want to speak to Red but he tells her that he can identify the man responsible for the attack. He asks her to meet.
At a warehouse, Elizabeth meets with Red and Dembe. Red tells her the attacker is a research scientist named Frederick Barnes. He was a genius savant of mass-killing. He has been selling his skills to the highest bidder. Red says that Barnes has the tools and know-how to kill thousands of people at once. Red doesn’t know why he has suddenly become the delivery-agent of his own weapons. Red says now that he is willing, Barnes is the most dangerous man in the world.
At a nearby hospital, Ressler and Elizabeth speak to the doctors about the victims. They died of Kurz disease, an extremely rare vascular disorder. The thing is, Kurz usually runs its course over the span of a decade, not two minutes. Elizabeth notes that some of the victims were children. The doctor says that someone has been intentionally spreading Kurz for months, on a smaller scale. Now that person has found a way to weaponize Kurz in a fast-acting formula.
Red believes they can trace Barnes via the radioactive materials he used to pack the device. Red tells Cooper, Malik and Elizabeth that he has set a meeting with the man who is supplying Barnes with the material. Red says to meet this man, he needs to catch his plane. Red invites Elizabeth—he’s heading to Cuba. Elizabeth blows him off, still furious. Elizabeth says that she found Barnes’ old research partner and she’ll be interrogating her. Red explains again that he didn’t frame Tom; Tom is lying about the box. Elizabeth asserts that Tom is nothing but a harmless fourth-grade teacher. Red says they need to move past this because without Elizabeth working with him, Red is no longer interested. Elizabeth leaves without a word. Red departs with Dembe and his female associate. The woman tells Red that an item he has been coveting has gone on the market. Red gets excited and tells her to go ahead with their offer.
In Maryland, Elizabeth and Ressler arrive at the home of Forrester, Barnes’ old research partner. Forrester explains that Barnes is brilliant and has no idea why he left his position. Forrester and her husband cooperate with Elizabeth and Ressler, none the wiser about Barnes’ criminal activities. Forrester’s son, Ethan, enters to ask a question; Elizabeth notices that Ethan’s veins are prominent beneath the skin. Elizabeth asks Forrester if Ethan has Kurz disease—he does. Elizabeth tells Forrester that Barnes used Kurz to murder thirty-seven people with Kurz and that is too big of a coincidence. Forrester whispers to Elizabeth that Barnes is Ethan’s real father and that her husband has no idea. Ethan developed Kurz at age five and Barnes researched it obsessively ever since. He couldn’t get funding to research Kurz because it’s too rare. Elizabeth deduces that Barnes is murdering with Kurz in order to spur funding into Kurz disease research.
In Havana, Cuba, Red embraces an Hispanic man. Red converses with an elderly woman in Spanish; she gives him a snack. Red meets with Manny, a large man with burn scars on his face, and embraces him. Manny asks Red what he wants with this specific radioactive item, strontium 90. Red says he needs 12 kilos but doesn’t tell him why. Red insists that he needs it now; Manny says he needs three weeks. Red threatens to reach out to his previous buyer to make that person an offer—the previous buyer is Barnes. Manny gets offended and his guards approach.
“What—are they all going to draw their guns and hold them sideways, at me?” Red asks. Red surreptitiously plants a device under Manny’s table. Red tells Manny that he will pay the previous buyer ten million dollars to obtain the strontium 90 immediately. Red returns to Dembe and tells him that Manny should be calling Barnes any minute. Manny calls via his cell phone and the device under the table taps into and traces the call. Red calls Elizabeth and gives her Barnes’ location: Arlington, Virginia.
In a courthouse in Arlington, Barnes walks with another briefcase. He enters a crowded courtroom and places his briefcase on the floor. Barnes makes an excuse to exit, leaving the briefcase on the floor. Ressler and Elizabeth rush into the courthouse as Barns tip-ties the doors of the room where he planted his device. Elizabeth cleverly pulls the fire alarm; as everyone begins to leave, Barns hits his remote control. The briefcase begins spewing smoke and those in the courtroom are trapped. When the fire department arrive, Ressler takes a mask and oxygen tank before heading into the court room. Everyone is dead on the floor, their arteries having exploded.
Meanwhile, Elizabeth spots Barnes in the crowd and chases him. Barnes grabs a gun from a security guard and forces Elizabeth to drop her gun, or he will kill a hostage. Barnes shoots the glass from the windows above, causing panic; he escapes in the chaos.
Inside, Ressler finds a single survivor of the attack in the courtroom—a young woman.
In Maryland, Red’s associate Lew greets a real estate agent. Lew declines a tour and offers double the asking price in cash, on the spot. The real estate agent is stunned.
At headquarters, Elizabeth and Ressler discuss the survivor of the Kurz attack. Malik determines that Barnes is targeting large, random cross-sections of the population. Malik suspects that Barnes has been experimenting, trying to find someone immune to the Kurz attack. Cooper interrupts and challenges Elizabeth; she gave up her weapon in a hostage situation and she will go under formal review. She let a mass murderer go free. Elizabeth asks Ressler why he reported her; Ressler says that Elizabeth is unfit to be in the field, given how she responded. Ressler says she traded one man’s life for possibly thousands of others.
In the hospital, the doctors draw blood from the one survivor of the Kurz attack. As the doctor exits with the blood, Barnes enters the survivor’s room, dressed as an orderly. Barnes changes the I.V. bag and the girl swiftly passes out. Barnes then opens a case containing surgical tools.
On his jet, Red gets a call from Elizabeth. Red tells Elizabeth that she can’t just call him whenever she needs something. Elizabeth tells Red that Barnes is out there because she let him slip away. “I need your help,” she says. Red makes her repeat that phrase. Red tells Elizabeth that Barnes will be going to visit the survivor.
Elizabeth and Malik rush into the survivor’s hospital room. They find her unconscious, having gone through a bone marrow biopsy. Barnes is using her genetic immunity to synthesize a cure for Kurz. Barnes will be headed for Ethan Forrester, his son.
In Bethesda, Forrester is doing the dishes when Elizabeth calls her. As Forrester is on the phone, Barnes enters. Barnes says he needs to see Ethan. Forrester questions Barnes about the murders. Barnes says he did what he had to do to save their son. Ethan enters and Barnes asks Ethan to trust him. Ethan flees upstairs and Barnes pursues him. Barnes prepares chloroform to knock Ethan out as Elizabeth and Malik arrive. Barnes chloroforms Ethan and the boy is unconscious. Elizabeth and Malik enter the house, guns drawn. They find Forrester on the kitchen floor. Elizabeth heads upstairs and draws her gun on Barnes. Barnes tells Elizabeth he has to do this to treat Ethan; Elizabeth says she’ll never let Barnes inject him. Barnes begs and says the syringe is Ethan’s only chance. He lunges to inject his son and Elizabeth shoots Barnes several times in the chest.
Later, ambulances are at the Forrester home. Red arrives and greets Elizabeth. Elizabeth asks why he’s there; Red gives her a treat from Cuba. Red says it’s a pity that Barnes is dead. Red says “someone who is willing to burn the world down to protect the one person they care about… that’s a man I understand.” Elizabeth says she doesn’t need Red’s protection, but she needs Red to do this job. She calls Red her partner but says she doesn’t want Red in her personal life in any form. Red says that he could disappear and cease to exist in sixty seconds; he’s not going to beg her for the privilege of helping her. If she tells him to leave, he will. Elizabeth says nothing. “Then I guess I’ll see you tomorrow,” Red says. He leaves.
At headquarters, Ressler tells Elizabeth she made a difficult call with Barnes. Elizabeth wonders what would have happened had she let him go through with it. She stands by her decision, even if it’s too late for Ethan.
Elizabeth returns home to find Tom on the floor, with all the furniture gone. Tom reminisces about when they first moved in and had no furniture yet. They share a romantic moment and begin to kiss on the floor, knocking over the one remaining lamp.
Back in Maryland, Red arrives at the house he had Lew purchase, with Lew and Dembe accompanying him. They walk into the empty house and Red looks around. “I remember it being bigger,” he says. Lew asks why he needed this house. Red says he raised his family there.
“It’s lovely,” Lew says.
“No it’s not; it used to be.” Red responds.
Red rips up a piece of a doorframe to find pencil marks chronicling his daughter’s growth. He looks out the window and remembers seeing his little girl playing in the yard. Dembe enters and tells Red it’s time to leave.
“Did you prepare everything the way I asked?” Red questions. Dembe nods. Red tells Lew that he spends every single day trying to forget what happened in that house. As they drive away, the house explodes in spectacular fashion.