From ABC's official press release:

The mystery of Izzie's strange behavior is unwittingly solved by her interns, as Derek falls into a dark depression and quits, on ABC's Grey's Anatomy.

Loretta Devine (Eli Stone and Boston Public) returns as Adele Webber.

After learning at a deposition that more of his patients have died than survived, Derek decides to quit, even as Meredith refuses to give up on him. Meanwhile Bailey and the Chief continue to bicker over her peds fellowship -- until Adele steps in to mediate -- and Izzie's interns discover something unsettling on a "patient x," unaware that the charts they're looking at are actually Izzie's, on Grey's Anatomy, Thursday, March 12 (9:00-10:02 pm, ET) on the ABC Television Network

Grey's Anatomy stars Ellen Pompeo as Meredith Grey, Patrick Dempsey as Derek Shepherd, Sandra Oh as Cristina Yang, Katherine Heigl as Isobel "Izzie" Stevens, Justin Chambers as Alex Karev, T.R. Knight as George O'Malley, Chandra Wilson as Miranda Bailey, James Pickens, Jr. as Richard Webber, Sara Ramirez as Callie Torres, Eric Dane as Mark Sloan, Chyler Leigh as Lexie Grey and Kevin McKidd as Owen Hunt.


Source: ABC
(Image courtesy of ABC)