The penultimate episode of “Criminal Minds” Season 12 delivered some major shockers. Although fans were led to believe that the notorious serial killer Mr. Scratch (Bodhi Elfman) was behind Dr. Spencer Reid’s (Matthew Gray Gubler) incarceration and the kidnapping of his mother Diana (Jane Lynch), episode 21 revealed that it was actually Cat Adams (Aubrey Plaza) who orchestrated the two events.

In fact, Cat worked alongside her accomplice Lindsey Vaughn (Gia Mantegna) to implement Diana’s kidnapping and Reid’s arrest for killing a doctor in Mexico. Now, with only one episode left to wrap up all loose ends, the FBI’s Behavioral Analysis Unit (BAU) has their work cut out for them.

Here is everything you need to know about episode 22, titled “Red Light”:

1. Synopsis

The synopsis of Season 12, episode 22 on TVGuide states that Reid will be caught in an impossible scenario and will be forced to square off with an old nemesis. Derek Morgan (Shemar Moore) will return to the FBI with a lead that might just help them catch Mr. Scratch once and for all. Jane Lynch also returns as Diana.

READ: Check out photos of the “Criminal Minds” cast having fun on set.

Moreover, Morgan’s lead could not only help them catch Scratch but also kill him, according to an article by CBS.

2. Derek Morgan Is Back!

One of the most exciting things about the final episode of “Criminal Minds” Season 12 is that Moore will reprise his role as the tough-as-nails former FBI Agent. Showrunner Erica Messer talked about the fan-favorite character’s return to TVGuide. “Derek Morgan is a civilian now, so he doesn’t look like his agent Derek Morgan self. He’s a dad, he has a beard, he’s a little more casual.”

Unfortunately, Scratch has got to Morgan. Messer explained, “Scratch isn’t getting to Hotch (Thomas Gibson) to get us that message because Hotch is in Witness Protection, but he did get to Morgan even though Morgan is a civilian now. That’s still a little disheartening that Scratch can have so much reach.”

3. Mr. Scratch Is A Huge Part Of The Finale

Although Cat was revealed to be the one who got Reid in prison, the mind-controlling serial killer will still play a crucial role in the finale. Messer told TVLine, “Scratch will definitely still be playing a part, all the way to the last frames of the finale. Just when we think we’ve got a handle on things, you get another curve ball.”

4. A Surprising Guest-Star

Speaking of curveballs, the guest cast list of the upcoming episode reveals the return of a surprising character: Reid’s dead lover Dr. Maeve Donovan (Beth Riesgraf). Of course, it is entirely possible that Maeve could appear as a part of Reid’s hallucination but it would be nice to see the nerdy profiler get a moment of happiness by reuniting with his one true love.

5. The Season 12 Finale Ends On A Cliff-Hanger

Unfortunately, “Criminal Minds” won’t end by tying up all of its loose ends. Messer said, “It isn't a car exploding [like in Season 3], but it is a legit cliffhanger that we haven’t done in a really long time, so that’s kind of exciting. We get to start Season 13 in a different way. You’ll be asking the same type of question: ‘Who was that?’ ‘Are they OK?’”

“Criminal Minds” Season 12, episode 22 airs Wednesday, May 10 at 9 p.m. EDT on CBS.