Event One is what they call The Big Bang
in the Doctor Who
universe. It warrants mention because it is one of the only things powerful enough to destroy the normally indestructible TARDIS
Event One in Doctor Who was (or is, time travel is involved) caused by the explosion of one of the fuel pods from the gigantic space station and time machine Terminus, which triggers the chain reaction that starts the universe.
It is here we must note that the Doctor Who radio play Slipback features a similar event, when the starship Vipod Mor travels back in time and explodes. This error in continuity is easy to explain: The Vipod Mor hits the fuel pod.
Event One features in the following stories: