Doctor Who Series 9: 22 Easter Eggs You Need To See

2. Four Knocks

"It's always four knocks." The Doctor hears that familiar sound in the series finale Hell Bent, a throwback to a couple of previous episodes from the David Tennant era. The first is the sound that is constantly in the Master's head (the Jon Simm version, anyway), a four-beat pattern that represents two heartbeats. More poignantly, it's a callback to Wilf's knocking on the radiation chamber in The End of Time. The sound is significant because it was part of a prophecy foreshadowing the Doctor's death, which he at first took to mean the Master, but later realised it meant saving Wilf's life at the cost of his own.