Red tells Liz about the next Blacklister, The Alchemist, who protects the guilty by preying on the innocent, using science to transform his marks. Red believes he has been contracted to "protect" Serbian mob informant Pytor Madrczyk and his wife, Catherine. After being seduced by a man named Eric Trettel, Sarah Jenkins wakes up on a private Gulfstream wearing a wedding ring, her physicality having been radically transformed overnight. The man next to her with Pytor's passport in his pocket is shocked to see his arms covered with Slavic tattoos. The flight attendants are dead, and within moments, the pilot parachutes out the cabin door, leaving the plane to nosedive...
Cooper demands to know how Red predicted the Madrczyks would disappear. Now they're burned beyond recognition from the plane crash. Liz informs the team that Red refuses to come into the Post Office while there's still a mole on the loose. Insisting there's no mole, Cooper sends Liz to find Red, who's hired a group of young whistle blowers to reconstitute hundreds of bags of shredded classified government documents in hopes of finding the FBI's mole.
Red tells Liz that Madrczyk isn't dead; the Alchemist must have abducted someone and altered his DNA to match Madrczyk's for a positive ID on the corpse. Since Madrczyk tried to hire Red, Red knows he's in Budapest, which is where Liz, Meera and Ressler capture him. Back home, Tom tells Liz he decided not to interview for the job in Oklahoma. Their surrogate is due in six weeks, so are they still adopting her baby? Tom isn't asking Liz to quit her job, but he is asking her to be more present and less secretive. Oh, and one of the teachers from his school is throwing them a baby shower, and they're hosting...
Red isn't sure how the Alchemist manages to get his clients' DNA into his victims, but one thing's for sure, he's manipulating genetics to fake his clients' deaths. Innocent people lose their lives so the guilty clients can live. Aram manages to positively ID Sarah Jenkins based on a bone marrow sample. Three days ago, Sarah was using an online dating website, which is presumably where she tangled with the Alchemist.
A furious Catherine Madrczyk pays Trettel to visit. What happened - why did the FBI capture her husband in Budapest!? After learning the name of the Madrczyks' attorney, Trettel shoots Catherine dead. The next day, Madrczyk tells Ressler that he never met the Alchemist; all business, as well as an exchange of DNA samples and dental records, was conducted through couriers. Just then Trettel arrives, posing as Madrczyk's attorney and asking for a moment with his client. Without cigarettes, Madrczyk is cranky, so when Trettel pulls out some anti-smoking gum, he demands a piece. Moments later, he's doubled over, just as Liz shows Ressler a photo of Sarah Jenkins' date: Eric Trettel! Ressler runs for the interrogation room, but Madrczyk is dead, and Trettel is long gone.
Meera quickly assembles a file on Trettel, a disgraced DNA expert who fell off the grid, leaving behind a wife and daughter. Liz visits with Molly Trettel and her young daughter, Annie, who wears an insulin pump, having recently been diagnosed diabetic. Molly is clearly scared of her crazy husband, who disappeared when their daughter was two but still thinks of them as a family. That night, Meera is called to a crime scene. The Alchemist's burned-out car contains the remains of a corpse matching Catherine Madrczyk and a slip of paper with a serial number to a piece of lab equipment. Little do they know, Trettel is reeling in his next mark, a young actress named Mandy.
That night, Liz can't help but enjoy herself at the baby shower. The partygoers are stunned to learn she has no intention of taking time off work when the baby comes. When the conversation morphs into argument, Liz and Tom retreat to the bedroom, just as a pretty young woman steps out of the bathroom to introduce herself as Jolene. Liz gets called into work, leaving Tom alone with Jolene, who has plenty of friendly relationship advice. Maybe Tom should bring Liz to an art exhibit tomorrow night? Meanwhile, Trettel is hard at work in his lab, transforming an unconscious Mandy into someone else.
The FBI finds Trettel's deserted lab, which contains evidence to nail him for several murders. Liz immediately worries for his family - whose home is already a crime scene containing two dead bodies. The next morning, Liz calls Tom from work to apologize and ask if he can cook her dinner that night so they can talk. That's when she realizes Molly and Annie Trettel aren't dead... Annie's insulin pump, which uses radio frequency, leads the FBI to Trettel, who's at a gas station/mini-mart 70 miles away. Just as he realizes the insulin pump will be his undoing, Annie's photo appears on TV and the attendant calls 911. Trettel pulls out a gun and starts shooting, hitting Annie in the process, just as a SWAT team pulls up outside.
By the time Liz and Ressler arrive, SWAT is preparing to shoot Trettel, so Liz puts up her hands and starts yelling. Trettel engages with Liz, hoping to negotiate his immunity, which is when his wife shoots him dead. Of course, Liz misses dinner with Tom, who's just about had it. Rather than return home, she proceeds to the synagogue with a list of people Trettel disappeared. When Red pockets the list without looking, Liz figures he sent the FBI after the Alchemist just so he could have the list. All Red will say is he has other business to attend to...
Tom calls Jolene and heads to the photography exhibition, so he's gone by the time Liz returns home with takeout and doesn't pick up her calls. With a pistol in his lap, Red's waiting for Meera when she gets home. Meera knows he's come to kill her... because she's the mole!
The FBI's next target protects the guilty by preying on the DNA of the innocent. Ryan O'Nan guest stars.