Benedict Cumberbatch Faces His 'Archnemesis' on PBS' 'Sesame Street' (Video)

Benedict Cumberbatch with the Muppets on "Sesame Street"

The lines are blurred for the "Sherlock" star when the Muppets start calling him a different name.

Benedict Cumberbatch dropped by PBS to help the Sesame Street Muppets with their latest lesson.

With the help of Count von Count and Murray Monster, the Sherlock and Star Trek Into Darkness star — who also appears in 12 Years a Slave and August: Osage County — helped figure out which is more: four apples or three oranges. But of course, there had to be some Sherlock Holmes jokes.

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For instance, meet "Murraryarty," Cumberbatch's self-proclaimed "archnemesis." When Cumberbatch tries to tell Murray that he doesn't really have archnemeses "in real life," Murray pretends not to listen: "Well, you do today!"

In another endearing moment, Murray can't seem to wrap his head around the fact that Cumberbatch isn't actually Sherlock Holmes, calling him "Benedict Sherlock" instead.

Sherlock closed out its three-episode third season in the U.S. with 3 million viewers this past Sunday against the Super Bowl.

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Though nothing official has been announced, Sherlock co-creator Steven Moffat joked in January during the biannual Television Critics Association press tour that the show would "continue until Benedict gets too famous," with season-four planning already under way.

Watch the clip below.