The TV origin story around a vigilante serial killer just recruited a former vampire killer.

Paramount+ announced Thursday that "Buffy the Vampire Slayer" standout Sarah Michelle Gellar has joined "Dexter: Original Sin" as "a special guest star." Geller joins the cast of the sequel series that includes previously announced stars, Christian Slater and Patrick Dempsey.

Gellar, 47, will play Tanya Martin, the CSI Chief at the Miami Metro Police Department in the origin story series about Dexter Morgan, everyone's favorite serial killer who's just trying to do right. Michael C. Hall famously played the police blood-spatter expert for eight seasons of Showtime's "Dexter" and in the spinoff "Dexter: True Blood."

You've no doubt figured out that Gellar will be the boss of young Dexter (played by Irish actor Patrick Gibson) in "Original Sin." The budding serial killer and college graduate begins putting his deadly talents to good use with a forensics internship at the Miami Metro Police Department in the new series.

It's quite a McDreamy police station, with the reigning Sexiest Man Alive! Dempsey, 58, playing Aaron Spencer, captain of Miami Metro Homicide. 

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What is Christian Slater's role in 'Dexter: Original Sin'

Slater will play Dexter's father, homicide detective Harry Morgan, who has a close relationship with Spencer. Morgan tries to guide his troubled adopted son into the light, instilling a code of honor and rules for his killing. The father had died before the original "Dexter" series. But James Remar played him in his son's loving flashback scenes.

What is 'Dexter: Original Sin' about?

The series will travel back to Miami in 1991. "Original Sin" follows Dexter (Gibson) as he transitions from college student to avenging serial killer.

"When his bloodthirsty urges can’t be ignored any longer, Dexter must learn to channel his inner darkness," Paramount+ says in a statement describing the series. "With the guidance of his father, Harry (Slater), he adopts a Code designed to help him find and kill people who deserve to be eliminated from society without getting on law enforcement’s radar. This is a particular challenge for young Dexter as he begins a forensics internship at the Miami Metro Police Department." 

When does 'Dexter: Original Sin' come out?

Paramount+ has not announced a streaming date for "Original Sin."

Is Dexter's sister in the prequel series?

Dexter’s younger sister, Debra Morgan, will be featured in "Original Sin," played by "Senior Year" actress Molly Brown. Debra, who also worked at Miami Metro Homicide in the original series, was played by Jennifer Carpenter back then.

Other OG characters are coming back as their younger versions as well for "Original Sin."

• James Martinez will play Angel Batista, a police friend, stepping in for fedora-wearing David Zayas in the original series.
• Christina Milian plays Maria LaGuerta, Miami Metro’s first female homicide detective, who rose to captain in the original series (played by Lauren Velez).
• Alex Shimizu is set to play Vince Masuka. The witty friend and fellow forensics investigator was played by C.S. Lee in "Dexter."
• Reno Wilson appears in the new role of Bobby Watt, the longtime partner and confidant of Dexter’s adoptive father, Harry. 

Will Michael C. Hall appear in the new 'Dexter'?

Hall, who starred in the original "Dexter" series starting in 2006, does not have a regular role in the "Original Sin" series set 15 years earlier. Hall is an executive producer on the new series along with Clyde Phillips, who also oversaw the original 2006-13 series.