'Criminal Minds' season 11 spoilers: BAU questions a serial killer about two kidnapped boys in episode 21

(Facebook/CriminalMinds)"There's a storm coming" on the next episode of "Criminal Minds" season 11.

Dr. Tara Lewis will provide some much-needed insight for the Behavioral Analysis Unit as they face a serial killer that one of their own had put away.

The BAU can't seem to get away from the damages that a flawed social services system creates. After dealing with an UnSub who was placed in an abusive foster home in the previous episode, the profilers will be revisiting an old UnSub who took advantage of a social problem to feed her compulsion.

In episode 21, "Devil's Backbone," an intercepted prison package reveals evidence that's related to two boys who were kidnapped years ago. This prompts the BAU to question the infamous Antonia Slade, a serial killer that the team's very own Jason Gideon (Mandy Patinkin) put away 15 years prior.

"Titanic's" Francis Fisher plays Slade, a criminal mastermind who set up a runaway hotline for teens and who severely punished those teens who misbehaved. "Criminal Minds" showrunner Erica Messer was enthusiastic to TV Line about Fisher's guest star turn.

"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," Messer explained.

A trailer for the upcoming episode gives viewers a glimpse of why the showrunner was so excited to have Fisher on board. The preview starts with an older woman being transported, and later, talking to someone from inside her cell about some missing boys. A scene showing 2 boys flashes on the screen, with one boy apparently sick, while the older one tends to him. Dr. Lewis (Aisha Tyler) tells Hotchner (Thomas Gibson) and the others that Slade is "orchestrating the entire thing from behind bars."

That assessment seems to ring true as she tells the BAU that the boys are still alive. But she also hints that they won't be alive for much longer. Hotch decides that they have to give her what she wants, and what she apparently wants is to communicate with Reid (Matthew Gray Gubler). The seasoned profiler goes to her and she seems to take his hand. The clip ends with a warning from her that "there's a storm coming."

What this storm is anyone's guess, but it might be a hint as to what's coming in the show's season finale. According to reports, the final episode of "Criminal Minds" season 11 will find the BAU scrambling to prove Hotch's innocence when a SWAT team apprehends him for conspiracy.

The newest episode of "Criminal Minds" season 11 will be aired on Apr. 20 at 9 p.m. on CBS.

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