Still feeling responsible for Don's near-fatal stabbing, Charlie puts his focus into tracking down an unnoticed serial killer, with the agents revisiting math theories from when Charlie first started working with the team to help solve the case.

When Amita is abducted, Don and the team's frantic search for her leads them to the delusional head of a cult, but it is up to a distraught Charlie to discover what the suspect's bizarre plans for her are.