Doctor Who is quite tired after his adventure with the Cybermen, so he turns into someone else for a bit. See that guy there, with the dark hair? That's Doctor Who now.
What's great is, the programme makers didn't even bother to explain this astonishing development. They just went "Doctor Who is this guy now. Get used to it. Look, Daleks!"
And everyone went "Ooh! Daleks!" Because Daleks are fun.
The Dalek here is pretending to be nice. "I'm fine!" he's saying. "I like humans and being nice and not killing humans."
Doctor Who doesn't believe the Dalek at all. "Shut your lying whore mouth!" he shouts.
The Dalek is quite taken aback at this. "What's got into Doctor Who?" he thinks, "And why is he wearing that hat?"
Look at Doctor Who's stupid hat. That's another way they tried to stop us worrying that it was a different guy. People just looked at the hat and went "Why is that hat happening?"