This week:
A hideous, misshapen creature releases a butterfly.
Next week:
The consequences of this simple action ensure that history follows its predicted path...
Sometime:
In the swirling maelstrom of the Time Vortex, The Council of Eight map out every moment in history and take drastic measures to ensure it follows their predictions.  But there is one elemental force that defies their prediction, that fails to adhere to the laws of time and space...  A rogue element that could destroy their plans merely by existing.

Already events are mapped out and defined.  Already the pieces of the trap are in place.  The Council of Eight already know when Sabbath will betray them.  They know when Fitz will survive the horrors in the Institute of Anthropology.  They know when Trix will come to his aid.  They know when the Doctor will finally realise the truth.

They know this will be:
Never.

This ia another in the series of adventures for the Eighth Doctor.