The Behavioral Analysis Unit (BAU) interrogates a serial killer about the abduction of two boys in the upcoming episode 21, titled "Devil's Backbone."

The security guards at a Virginia prison receive a package from an anonymous person. They discover that it contains evidence that has connection with the kidnapping of two boys several years back. The BAU interrogates an infamous serial killer, Antonia Slade, who possibly holds the key about the boys' whereabouts.

The promo trailer reveals that the notorious serial killer, who is in prison, is the mastermind of the boys' abduction. BAU also finds out that she orders the crime from behind bars. She is asked if they boys are still alive; she confidently responds that not for long. Aaron suggests to give her what she wants.

Frances Fisher will play as the vicious serial killer Antonia in the penultimate episode of the show's current season. Showrunner Erica Messer described her character as "a crazy UnSub who was put away by [Mandy Patinkin's] Jason Gideon 15 years ago."

Messer told TV Line that Antonia and her husband had created a runaway hotline years back for teenagers. However, they abduct those teens and severely give punishment to those who did not behave. She further revealed that the story will be interesting, "creepy and psychological."

"She's amazing," Messer said about Fisher. "We've been wanting to work with Frances Fisher for years and it never worked out, and we're so glad it didn't, because she was made to be this UnSub for us."

Meanwhile, the previous episode, titled "Inner Beauty," showed the BAU hunting down a serial killer in Sacramento, California who is known to disfigure his victims before the killing them. Rossi, on the other hand, ended up enduring an awkward reunion with his second former wife Hayden. She was disappointed at him because he wasn't there for her during their entire marriage.

"Criminal Minds" season 11 airs every Wednesday at 9 p.m. on CBS.