Benedict Cumberbatch sleuths with Muppets in PBS video

Benedict Cumberbatch may play super-sleuth Sherlock Holmes, but he needed a bit of help cracking this case.

The British actor of "Sherlock" and "Star Trek Into Darkness" fame appears with a pair of "Sesame Street" Muppets in a video posted Tuesday on PBS' YouTube channel.

With the help of Count von Count (Matt Vogel, who replaced the late Jerry Nelson) and Murray Monster (Joseph Mazzarino) Cumberbatch explains how to count up four apples and three oranges, and which is the greater number.

He also, patiently and repeatedly, tries to get a skeptical Murray -- who calls himself Sherlock's arch-nemesis, Murray-arty -- to believe that he is not actually the famed detective, but just plays him on TV.

Watch them unravel the "mystery" below: