Conrad's ex-fiancé, Catherine, is admitted to the hospital with abdominal pain that proves to be quite the medical mystery. As Conrad doggedly tries to figure out her diagnosis and fights the idea of Bell performing her operation, Nic gets some insight into Conrad's past. Meanwhile, Bell panics when Claire announces that she will be installing cameras in all operating rooms, Devon wracks his brain for the perfect birthday present for Priya and Lane realizes that Nic is suspicious about her clinic.

In a final and drastic attempt to save Nic from Lane's acts of sabotage, Conrad must call in a favor from the one person he promised he never would. Meanwhile, Mina's partnership with The Raptor takes on new colors when Micah is rushed back to Chastain with major heart complications and Dr. Bell's alliance with Lane is put to the ultimate test when one of her patients is readmitted.

Halloween at Chastain proves to be just as spooky as ever when Conrad is tasked with diagnosing a young woman admitted to the hospital with extreme night terrors. Meanwhile, Devon faces his first Halloween in the ER, Nic panics over her decision to bring her newly sober sister to Mina's epic annual Halloween party and Bell finds an interesting way to make use of his new assistant.

The new neurosurgery center opens, and Nic is forced to work alongside Cain again. Mina and The Raptor work together with a new surgeon on a heart transplant that has an unexpected result. Bell asks Conrad to reexamine three patients who may have been poisoned.