Believe it can be done. When you believe something can be done, really believe, your mind will find the ways to do it. Believing a solution paves the way to solution.
David Joseph Schwartz
In this two-part episode, The Impossible Planet and The Satan Pit, the Doctor and Rose find themselves on a planet at the edge of a black hole–a planet that shouldn’t exist. The team of scientists on this planet have been working on drilling into a mysterious cave under the planet’s surface and, while stuck on the planet, the Doctor and Rose decide to help. Meanwhile, the Ood, an alien species dedicated to a life of servitude, are acting strangely. Normally subservient and polite, the Ood under the command of this research team begin to strike out violently. Adding to this madness is the possession of one of the scientists by what appears to be a powerful creature from the cave.
They discover later that this cave is, in fact, the prison of Satan himself and that he has been trapped there since before the beginning of time. Several of these concepts prove challenging to the Doctor. As a Time Lord, he doesn’t believe that there was a time before the beginning of time. Even simply attempting to put the concept into words makes it sound ludicrous. He also struggles to accept Satan’s existence. At first, this disbelief is a disadvantage for the Doctor, as it leads to inaction and allows Satan to wreck further havoc on the group of scientists. But when the Doctor chooses to use the power of belief to his advantage, he is able to overcome situations that would otherwise seem impossible. When Rose finds herself in yet another precarious situation, the Doctor still cannot bring himself to believe in Satan. But, as he sees the destruction going on around him, he is able to bring himself to accept the existence of a force beyond his comprehension and uses his belief in Rose to help them both save the scientists. The Doctor discovers that it is not so much what you believe that matters, but the strength of your belief (in whatever seems true to you) that makes faith so powerful.