The BAU investigates a serial killer active in Bakersfield, California, who leaves unidentifiable victims behind. Also, the team welcomes new agent Kate Callahan into the BAU, who assists them in their efforts in solving the case.
- Daryl Crittenden - Mark
- Emily Dykes - Sandy Cowles
- Eric Frentzel - Steven Parkett
- Katie Hilliard - Angie Stanton
- Lily Mariye - Doctor Lee
- Kerr Smith - Frank Cowles
- Hailey Sole - Meg Callahan
- Al Vicente - Lieutenant Ted Banks
- Jakob Wedel - Corey Cowles
- Jeffrey Dahmer, The Milwaukee Cannibal
- "It's Not Right for You" by The Script
- "No Good in Goodbye" by The Script
- David Rossi: "I remained too much inside my head, and ended up losing my mind." Edgar Allan Poe
- Jennifer Jareau: "Just because everything is different doesn't mean anything has changed." Irene Peter
- Kate Callahan, Alex Blake's replacement, is introduced for the first time in this episode.
- X's air-date of October 1, 2014, marks the first time a season for the show has premiered in a month other than September.
- Curiously enough, X is Episode 10.1 of the season, and is scheduled to premiere on 10/1. Erica Messer explained that this was merely a coincidence, as the reason for the delay is to coincide the season premiere with a new show that will follow it at 10:00 p.m., as well as the fact that the final Wednesday of September (the usual air-date for the show's season premieres) was crammed with a 90-minute season premiere for Survivor and the season finale for Big Brother.
- X is the Roman numeral for 10. It is unknown if it is a coincidence or a subtle reference to the fact that this episode starts off the show's tenth season.