A man releases a poison gas into a train car that kills everyone inside. They are contaminated with a rare disease that hardens the victims arteries. Naturally, Kurz disease is a slow progressing disease that eventually leads to death. In this case, the victims die within minutes. It's because the perpetrator uses Strontium-90 to weaken their immune system.
Red identifies the killer as a scientist, Frederick Barnes. Red goes to Cuba to track down Barnes through the Strontium-90 seller in Cuba, while Liz questions Barnes' former lab partner. Liz uncovers that Barnes and his lab partner had a son together who has Kurz disease.
Barnes attacks a court room and Liz almost captures him, but lets him go to protect his hostage. She's reprimanded for not following procedure. Ressler also admonishes her for protecting one life over the potential death of many.
During that attack, one woman survives and Barnes takes her bone marrow to try and save his son. Liz shoots Barnes before he's able to give the treatment to his son.
Red and Liz have a conversation about their working relationship. Liz wants it to remain professional and for him to stay out of her personal life. He says it's all or nothing. That she will want him to answer questions that she doesn't know yet. When Red threatens to walk out, she silently agrees to his terms.
Red's associate, Luli, buys a house for Red. He visits it uncovers growth markings on the wall and remembers a young girl playing in the yard. It appears it was his house with his family. He leaves and his associate blows it up.