A major security breach in the city's computer networks forces Firehouse 51's senior leaders to take matters into their own hands. Severide gears up for an arson investigation.

With Astra, Spooner and a now-human Gideon trying to save the Legends, Gideon becomes overwhelmed by her new human choices sending her into a catatonic state. Astra and Spooner combine their powers to enter Gideon's mindscape and discover that a virus is trying to erase all of Gideon's memories. Acting fast, they devise a plan to defeat the virus before it's too late. Meanwhile, the Legends are about to go up against a powerful new foe.

Ambrose heads to the mainland to learn more about Percy's past and uncovers dark family secrets.

Jimmy addresses a proposed tax on billionaires and a dangerous new UK dating show, then catches up with Elton John and Ava DuVernay before welcoming musical guest Walker Hayes.