The BBC have released the official synopsis for episode nine of the series which is called Cold Blood:
"It is the most important day in the history of the Earth; the dawn of a new age of harmony or the start of its final war.
In the latest episode of the time travelling drama, written by Chris Chibnall, the Doctor must face his most difficult challenge yet. It is a battle in which he can not take sides and a day when nobody must die..."
The episode will be shown on Saturday 29th May at 6.25pm.