Something sparks in the Doctor at the words, a –bright and painful– barb of defiance shooting toward the Master, but it fades as quickly as it came. Because the other is right, and there is no victory for the Doctor here no matter what choice he makes.

Also, he’s forgotten what he was trying to do in the first place.

So he lets go, eases back and allows the Master’s presence. I’m sorry, he says, and then, Master. and then, please. Slowly, and with a little reluctance, he opens a corridor in his mind, inviting the other wordlessly to follow.