Doctor Who: Why Each Doctor Was Forced To Regenerate

The Sixth Doctor's regeneration is easily the most bizarre of the lot, and that's because we don't really know what caused him to regenerate.

This is because of the behind-the-scenes drama surrounding actor Colin Baker, who was fired from the role, and as a result, refused to return to film his regeneration. As such, the production team couldn't properly depict the Sixth Doctor's death, since they didn't have Baker around to help them shoot these scenes.

The novelisation of Time And The Rani (the serial in which Six regenerates) does mention that he bashes his head against the TARDIS console, but in terms of the actual televised show... we don't get much information at all.

Time And The Rani begins with the TARDIS spiralling out of control, and when we cut to the console room, the Sixth Doctor - who is actually being played by Sylvester McCoy in a curly blonde wig - is already on the floor, dead. The camera then closes in on his body, his face glows, and the Seventh Doctor is born.

Baker has since gone on record saying that he regrets not filming his regeneration, but unlike the Doctor, he cannot simply hop back in time and do it all over again. Sadly.