To avoid falling into Event One (the Big Bang), the Doctor must convert mass into momentum by having the TARDIS jettison a quarter of itself. Unfortunately it can not be known which rooms will be ejected.

The Doctor discovers Castrovalva is a cursive occlusion, an impossible yet manifested Escheresque environment, with all pathways, no matter which direction one goes, leading back to the same point. It is a trap laid by the Master with no apparent way out.