“Elementary” has enlisted Owain Yeoman for its 100th episode.

According to TVLine, Yeoman — best known for his role as Wayne Rigsby on CBS’ “The Mentalist” — will guest-star in the milestone installment as Sherlock Holmes’ (Jonny Lee Miller) boarding school classmate, who will help the detective solve an apocalyptic case.

Series executive producer Rob Doherty told the news outlet that the episode, titled “Henny Penny The Sky Is Falling,” centers on the mystery surrounding the death of a man “who was looking for better and more efficient ways to identify what are known as Earth-killer asteroids.”

“So there’s mystery on top of mystery as to why anyone would want to kill someone who might be able to help NASA locate and, hopefully, divert a small planetoid hurdling towards Earth,” teased Doherty.

“It’s a fun and very colorful role,” Doherty added of Yeoman’s character, whose debut episode is set to air on Sunday, Oct. 23.

Besides “The Mentalist,” Yeoman’s other notable credits include AMC’s “Turn: Washington’s Spies,” CBS’ “Extant” and HBO’s “Generation Kill.”

Just last week, TVLine also revealed that “True Blood” alum Nelsan Ellis will play the recurring role of Shinwell Johnson — a character created by Sherlock author Sir Arthur Conan Doyle in his 1924 short story, “The Illustrious Client.”

After spending a long time behind bars on drug charges, Ellis’ contemporary version of the character is attempting to get his life back in track when his old acquaintance Joan Watson (Lucy Liu) asks him for help to solve a murder.

Shinwell will first appear in the Season 5 premiere where he leverages his illicit connections to help Joan with the case. Shinwell presumably does a great job because Joan convinces Sherlock to put him on the proverbial payroll.

After the season premiere, Ellis is set to appear in at least two episodes more.

As for returning characters, Entertainment Weekly revealed last month that Virginia Madsen will reprise her role as Captain Gregson’s (Aidan Quinn) love interest Paige Cowen in Season 3, Episode 3. “We’re going to see that relationship continue to evolve and advance,” Doherty told the news outlet.

“Elementary” Season 5 premieres Sunday, Oct. 2 at 10/9c on CBS.

Owain Yeoman “Elementary” Season 5 has cast “The Mentalist” alum Owain Yeoman as Sherlock Holmes' (Jonny Lee Miller) boarding school classmate. Pictured: Yeoman poses during a photocall for the TV show "The Mentalist" as part of the 2011 Monte Carlo Television Festival held at the Grimaldi Forum on June 08, 2011 in Monte-Carlo, Monaco. Photo: VALERY HACHE/AFP/Getty Images