The fuse has been lit. Reality has been blown apart, and the barriers that shield our universe from the endless others running parallel have shattered with it. The only chance the Doctor has of saving the multiverse from total collapse is if he can get back to Earth — where the damage was first done — and put things right.
With time running out, the Doctor finally understands why 'our' universe is unique. In proving it, he nearly destroys the TARDIS and all aboard — and becomes involved with the machinations of the mysterious Timeless organisation. They can fix your wildest dreams, get away with murder and bring a whole new meaning to the idea of victimless crime.
Soon, Fitz is married, Anji's become a mum, and an innocent man is marked for the most important death in the universe's long history. The reasons why force the Doctor into a deadly showdown with an old, old enemy, in a killing ground spawned before time and space began.