Doctor Who: Every Prophecy In The Show - Explained
4. The Prophecy Of The Four Knocks
As he got closer to his brush with the Immortality Gate, the Tenth Doctor had an adventure on the sandy planet of San Helios, along with a group of civilians.
One of those civilians was Carmen, a nice old lady who had a penchant for randomly spouting out disturbing prophetic statements. This is because of her mild precognitive powers, which allowed her to sense future events.
One of these events was the end of the Doctor's life, as foretold in her prophecy.
"Your song is ending, sir. It is returning. It is returning through the dark. And then, Doctor? Oh, but then... he will knock four times."
"He will knock four times" she tells him, with no further details given. Still, it's clear that she's referring to the person who will kill the Doctor, a prophecy that almost comes true in The Waters Of Mars, when an infected human gets to three knocks.
We then learn that the prophecy was indeed referring to the Doctor's killer, but in a cruel twist, it's actually one of his friends.
Wilfred Mott, trapped inside a radiation booth, taps on the glass four times and asks the Doctor to help him. The Doctor is then forced to swap places with Wilf, leading to the radiation poisoning that causes his regeneration.