In the apartment, Syd unlocks a crawlspace door and follows a passage to a room plastered with disturbing drawings. She's suddenly attacked and bitten in the throat by a feral Jason. Jack shoots him dead.

As Syd undergoes a battery of tests, the doctor says the Cahills came in contact with a drug that caused an acute melatonin imbalance, which incited their acts of extreme violence. Syd's diagnostics indicate that she's fine.

Sloane says Langley wants to find the drug, who manufactured it and what it's used for. Marshall traces the calls Nancy made and begins reconstructing erased files on her laptop. Weiss says Jason was trying to determine what he was infected with, even after Nancy locked him in the closet.

Syd can't sleep and begins hallucinating. Her temperature reads 111.

Jason's files indicate he had a meeting scheduled in Bucharest with "The Count," a drug supplier. Syd mission is to pose as a Cahill associate and meet The Count. Dixon tells Sloane it's a mistake to send Syd out so soon. Sloane harshly reprimands him.

In Bucharest, Syd has a recurring hallucination of a spider. Marshall calls Jack to say the drug's incubation period is a few days, like a virus, and permanently alters brain chemistry. Jack tells Vaughn that they should have her in sight at all times. At the meeting, Syd imagines Vaughn kissing someone. They take The Count into custody, and admits he recently delivered samples of a drug called "Nocturne" to Jason. Once the delusions begin, there's no stopping it. Syd, increasingly off-kilter, imagines Sloanee threatening her. Syd chokes The Count, who offers that the drug came from Prague and was an old sample meant to be destroyed.

Nadia finds a group called Grappig, which means "Tell them it's funny" - an affiliation of biochemical scientists in Prague led by Dr. Letuza. Nancy had found out where it came from but was too far gone. Vaughn says Letuza may have developed a remedy if he knew he made a bad batch. In Prague, Vaughn searhes for Letuza's files. Syd grows fearful that Jack will kill her. She asks to be tied up and Jack obliges. She imagines Jack comparing her to Irina. Syd head butts him, takes his knife and cuts herself loose.

Vaughn finds vials of antidote and returns to see Syd crying over an immobile Jack. She says she killed him, but it's a trap. She has a gun, forces him to his knees. She says their relationship is a cruel joke at her expense and that she's scared she will betray him again. She fires but the gun has no bullets. They fight. She beats him with a pipe. Jack wakes and subdues Syd with a sleeper hold. Jack had taken the bullets out! Vaughn administers the antidote.

In LA, Sloane says Interpol has raided Letuza's office and eradicated the Nocturne threat. Sloane admits to Dixon that sending Sydney was a tactical error. An angry and humiliated Dixon says he never saw the nature of SD-6 because his only failure was that of imagination -- he never imagined a person as thoroughly toxic as Sloane could exist. When Sloane's true motive reveals itself, Dixon promises to be there.

Syd and Vaughn kiss in her hospital room. She sees a spider on her bed. Vaughn brushes it away and says spiders are good luck.