When the entire universe is at stake, three different Doctors will unite to save it. The Tenth Doctor is hunting shape-shifting Zygons in Elizabethan England. The Eleventh is investigating a rift in space-time in the present day. And one other - the man they used to be but never speak of - is fighting the Daleks in the darkest days of the Time War. Driven by demons and despair, this battle-scarred Doctor is set to take a devastating decision that will threaten the survival of the entire universe. a decision that not even a Time Lord can take alone.