The one where Jo saves the day by offering to sacrifice herself...
It's an interesting idea that what we classify as magic is actually just a form of alien science that we don't understand. It sort of matches the belief that anything supernatural or paranormal is actually just the result of some kind of scientific discovery we have yet to make. It's an ingenious idea to have Azal awoken by psychokinetic power, the gathering of violent emotions such as greed and fear, which forms the basis of the Daemons' science. To us, psychokinesis is a paranormal mystery yet to be cracked, but to the Daemons, it's E=MC².
So you'd think that the Doctor might try and defeat Azal using the power of his own science, harnessing the collective psychokinetic energy of the villagers and UNIT personnel to fight back against him. But instead, he plans to use some dodgy gadget he's got Osgood to lash up, and which actually blows up before it can be of any use. The Doctor's very last resort is to just walk into the cavern and hope he can reason with Azal.