The Doctor and Rose arrive in a very odd place, where even the TARDIS can't translate certain writings they find; something the Doctor has never seen happen before. They find a crew of humans, working in a planetary outpost, who tell them they are investigating the mysterious planet's ability to remain in orbit around a Black Hole. Something that is quite impossible!
They find the human crew is attempting to learn the source of the ultimate power that keeps this planet out of reach of the forces of a Black Hole, which is drawing in surrounding solar systems. The power source of the planet appears to be located at the planet's center, under miles of hard rock.
But, then after one of many reoccurring earthquakes destroys the section of the base the TARDIS materialized in, the Doctor and Rose must accept that they may have no means of escape, or return to any other time or space.
And even worse, an evil force, and not technology, might be behind the planet's unexplainable power to orbit the Black Hole.