10 Doctor Who Moments You Didn't See Coming

Before we learnt its true meaning, the four knocks had been teased for quite some time. The first official mention of this prophecy was in 2009's Planet Of The Dead, but there's also reason to believe it was foreshadowed in 2008's Midnight.

Basically, we had to wait a long time for the truth to be revealed, and when that moment finally arrived, few could have predicted the heartbreaking circumstances.

The four knocks referred to the man who would kill the Tenth Doctor, and with David Tennant's regeneration just round the corner, we all knew that answers had to be coming soon. Throughout Ten's last two episodes, the smart money was on John Simm's Master - the main villain of the piece - being the one to perform those knocks. Or maybe it would be the powerful Rassilon. Or hell, maybe it would even be douchebag millionaire Joshua Naismith. As you can see, there were a fair few villainous candidates.

In the end though, it wasn't a villain at all. It was lovable old Wilf, Ten's loyal companion, tapping four times on the glass of a radiation booth. He had no idea what he'd just done, but us viewers knew he'd signed Ten's death warrant.

It was a stunning scene, and instantly ranked as one of the most emotional twists in the history of the show.