Not even Sherlock Holmes could deduce what character Benedict Cumberbatch is playing in J.J. Abrams‘ next Star Trek movie. Plot details of the film — which just finished shooting at Avatar‘s old soundstages in Playa del Rey, Calif., with the production now heading for San Francisco — are being kept double top secret. ”He’s not a good guy — that’s the most I can reveal,” the 35-year-old British actor says when asked about the role. There have been rumors he’s Khan, the genetically engineered superhuman Ricardo Montalban portrayed in the original series and in the 1982 film The Wrath of Khan. But we could easily see Cumberbatch playing any number of Trek baddies. Kodos the Executioner. The Gorn. One of those brains in a dairy case that made Kirk fight like a gladiator. “Diddly-squat” is all Cumberbatch will say. Of course, the actor, whose last big role was in Steven Spielberg‘s War Horse, doesn’t just play bad guys. He’s also starring as the title character on Sherlock, the BBC’s addictive reboot of Sir Arthur Conan Doyle‘s classic detective tales, which has Holmes and his sidekick, Watson (Martin Freeman, the guy who will play Bilbo Baggins in The Hobbit), solving crimes in contemporary London. Season 2 (if season is the right word, since it’s only three 90-minute episodes) starts airing on PBS’ Masterpiece May 6, although it was already broadcast in the U.K. earlier this year. “People went bonkers for it,” Cumberbatch says.