Brian Gideon, a disgruntled biophysicist, learns that STAR Labs is producing weapons of mass destruction based on his research for the United States government and uses a cloaking device to infiltrate STAR Labs and retrieve the weapons, exposing Tina and the project administrator Ruth to a deadly neurotoxin. While attempting to locate both Gideon and a cure, Barry clashes with federal agent Quinn, who is trying to cover up Gideon's--and Quinn's--involvement in the crisis.

James Jesse, a schizophrenic serial killer, becomes obsessed with Megan Lockhart, who had been investigating his activities, and attempts to kidnap her, but she is rescued by the Flash. Believing them to be romantically involved, Jesse adopts the identity of the Trickster and begins tormenting the Flash.