SIRENS BLARE! Castle is led down a tented corridor by two men in hazmat suits. He's sealed off in another tented section, but he's not alone. Beckett's there, too. An expression of stunned fear covers her face. It's a look that makes us think something terrible has happened.
We jump back 36 hours. The body of a dead cabbie is found near his stripped taxi. Amir Alhabi was an immigrant from Syria. He was shot in the head in what appears to be a robbery. Lanie says the victim's fingers were broken one at a time. Cell records show that his last phone call was to his wife. Also, Amir opened a notebook app and wrote down "C4121652" before he died.
Amir's wife, Nazihah, tells Beckett that her hubby was always worried about money because their daughter needed surgery. His cousin, Jamal, was Amir's business partner. He recently started his own moving company. The guy who stripped Amir's cab for parts says another man was in the area at the time of the murder. This person tore up the upholstery in the cab looking for something.
Kevin McCann, a driver who rents a shift with Amir's cab, had a little spat with his boss. McCann alibis out and says Amir had been flashing a lot of cash lately. A deposit for 10 grand was recently plopped into his bank account. Esposito learns that Amir had cameras installed in his cab. The hard drives that may have housed any recordings are gone. This case is starting to look like much more than just a simple robbery.
Amir gave a cab ride to Fariq Yusef, a member of the Syrian secret police. This has Castle spewing out all kinds of international conspiracy theories. But Yusef has an alibi for the murder. The team realizes the "C412" number Amir wrote down refers to a storage unit location. Castle and Beckett find a big crate housing plastic explosives. An alarm goes off. Beckett realizes she's been exposed to high levels of radiation. That's why she and Castle are quarantined by the hazmat team.
Castle makes small talk as he and Beckett await news as to the seriousness of their situation. The topic of discussion is Beckett's boyfriend Josh. He's headed for Haiti as part of "Doctors Without Borders" mission. Beckett says, "I wish that I had someone who would be there for me and I could be there for him and we could just dive into it together." Castle is about to respond when the hazmat folks say they are free to go. They'll be fine. Of course, they may have been even better had Castle been able to say what was on his mind.
Federal Agent Mark Fallon (guest star Adrian Pasdar) from the Department of Homeland Security takes over once the team learns that Amir worked on a nuclear weapons program for the Syrians. There was a second crate at the storage facility that housed a dirty bomb. And right now, it's missing.
Castle sends Alexis off on a spiritual retreat with Martha. His daughter isn't happy about it, but at least Castle knows she'll be safe and away from the city. As far as the case, Fallon presses Amir's wife Nazihah hard. Beckett doesn't believe she's a criminal, but she tells Fallon that she knows it's necessary to treat her as if she is. Another family member may also be involved when video surveillance shows Amir's cousin Jamal wheeling the missing crate out of the storage unit.
Unlike Agent Fallon, Castle believes Amir and Jamal may not truly be terrorists. He has an "off the record" chat with Fariq Yusef. The Syrian security man tried to convince Amir to come back the weapons program, but he wouldn't. Yusef then gives Castle his card, as he may soon need a man with his skills. When Fallon learns that Castle met Yusef with Beckett's approval, he boots them both off the case.
There's a citywide search for the truck holding the dirty bomb. Castle and Beckett continue their investigation on their own. Cabbie Kevin McCann was at the last two places Amir visited. That can't be a coincidence. The police find the missing truck, but the bomb is gone. That's because it's in a van at the warehouse that Castle and Beckett are investigating. BANG! BANG! BANG! A gunman fires on Castle and Beckett. They seek cover in a freezer as the van with the bomb drives away, heading right into the city.