Tim Curry, Criminal Minds

Bad things happen in Los Angeles when the lights go out.

That was the idea Criminal Minds co-executive producer Erica Messer and the rest of the writers kept in mind when crafting Wednesday's season finale (9/8c on CBS).

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"The mandate given to me by [executive producer] Ed Bernero for the finale was just to make it the scariest one we've ever done, which is an interesting assignment," Messer tells TVGuide.com. "I started thinking, 'OK we've done 114 episodes: What is the scariest episode we've ever done?' What I realized is, I needed to know what scares me. If I wrote my fear then there will be a truth to it."

So Messer added to that basic fear of the dark some elements of the serial killer Richard Ramirez, known as the "Night Stalker," who terrorized the Los Angeles area in the 1980s. "He attacked people in their homes, and that's where you're supposed to be safe," Messer says. "That's the one thing in this world you can trust, that you're safe in your home, and he took that away from everyone."

The end product is Minds' Billy Flynn, a serial killer who has for years preyed on victims during blackouts. Guest star Tim Curry brings the menace to life, and his casting naturally helped shape the role and the story.

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"Who else can be the creepiest guy without saying a word?" Messer says with a laugh. "Tim Curry is a dream. [The character] kills families, but always leaves one person alive, which is psychological torture for the rest of that person's life. ... He feeds off of making people powerless."

The members of the Behavioral Analysis Unit are no exception, particularly Morgan (Shemar Moore), who tries unsuccessfully to protect a young girl from Flynn. "The way that [Flynn] has been set up, you wouldn't want anyone to be around this man for a minute, let alone [have] a 10-year-old girl kidnapped by him," Messer says. "The idea that this little girl is missing should be the most disturbing thought. ... Emotionally, Morgan will have to live with some horrible things that happened on his watch. Maybe we'll be able to pick up that thread next season."

Maybe? Does that mean Morgan is in mortal danger? Messer doesn't rule out the possibility, but she insists this finale won't end with as many question marks as the previous two.

"We can't do that little trick again unless we're going to actually kill someone," Messer says. The decision Hotch [Thomas Gibson] rightly makes puts the entire city in jeopardy. ...The peril for the team is that there's now a city-wide blackout. Nobody knows where Morgan is at the end of the episode. There are gun shots in the dark, but the difference with this one is you'll know who got shot at the end of the episode."