Who Review - The Witch's Familiar

There are places the Doctor should never go. Planets where his life would not be worth an hour’s purchase. When he finds himself in the very worst of these, without his Tardis, or his sonic, and with his best friends murdered in front of his eyes, he has only his wits to keep him alive. And perhaps something else. What is the Doctor’s confession? Why did he really leave Gallifrey all those centuries ago? And is it a secret he is willing to give up?