House returns on April 11 with a milestone and a mystery.

The show will celebrate its 150th episode by asking a pressing question: where the heck has Thirteen been?

This preview for "The Dig" spoils the fact that the character has been in prison, so the better inquiry might be: Why? It's what House will try to get out of his underling for a majority of the hour.

“In order to sort of melt her steely resolve, he has to show a little bit of his own vulnerability, and I think that’s what makes their interaction really kind of unusual and really interesting,” Olivia Wilde tells EW of her return and interaction with House. “He’s reaching out to her and showing her his rawness... She eventually has to give it up.”

So... what was the basis for her incarceration? We can't say.

But Hugh Laurie can offer this intriguing tease: “I’d bet money no one will figure out why she’s in jail before we tell them because until I read it I had no idea. I think our fans will be left salivating.”