The Eleventh Doctor Chronicles Volume 5: Everywhere and Anywhere

Travelling in the TARDIS is filled with unimaginable delights. You can visit every wonder in the universe, meet the founders of a civilisation in the morning and their descendants in the afternoon. You get to witness the best of everything history has to offer. However, when you can go everywhere and anywhere, you also see the dark side of the universe. The cruelty and the hatred and the violence. The worst the universe has to offer must be fought.

That’s what the Doctor and Valarie do. They hold back the darkness, whatever the cost.