Whatever the Master did to win back his drums has torn apart his bond with the Doctor. They can no longer touch or be close without the drums, amplified by their minds, sending them into convulsions.
The Doctor decides to wander. A Time Lord is made with two hearts for a reason and without the Master, one has broken.
He meets another metacrisis, one that isn't as crazy as the rest, a grifter that tells him he's a pineapple on the scale of fruit, and finally, Not My Rose, who gives him a purpose again... They're going to stop the drums, or find a way to do it. First? Steal a TARDIS.