Although this is just rumour and has not been officially confirmed, potential spoilers ahead.
Doctor Who fans will be left on the edge of their seats during the closing moments of episode 11 of the latest series, when David Tennant bursts back onto our screens in a shocking 'degeneration' scene.
According to a source, the penultimate episode of Series 9 will see the Doctor discover Gallifrey hidden inside a magic castle in the middle of Wales. However, in an unexpected twist, the Time Lords intend to punish The Doctor by distorting his personal timeline for locking them away. The result of this punishment sees The Doctor 'degenerating' into many of his previous selves (apart from Christopher Eccleston, of course), and then settling on the body David Tennant, the tenth Doctor, before looking up to the camera and saying "I told you I didn't wanna go!" as he winks and grins at the audience.
And in another wibbly-wobbly-twisty-wisty, it is also alleged that Clara Oswald will be subject to a similar torture, and will be forced into degenerating into the form of Rose Tyler. The source also indicates that showrunner Steven Moffat is currently undergoing plastic surgery to turn himself into Russell T Davies.
Despite this, it is already confirmed that Peter Capaldi will be in the Christmas special with the big haired water-woman, so it is safe to assume that the degeneration will only last for one episode. Although what this means for Steven Moffat, we have no idea.