Though not entirely obfuscated in the first part of this two-parter - The Impossible Planet - the enemy here is none other than Satan/Lucifer/Prince of Darkness. What makes it clever here is that the writers don't really make it a connection to Christianity; matter of fact in this case, it's an entity who is "before time" - which, naturally, puzzles the Doctor immensely. The plot here continues from the first part where the Doctor and Rose, sans their Tardis which has fallen somewhere miles below on this planet which is being mined and is above a black hole, are stuck in a situation with a bunch of beleaguered, Aliens-esqe space crew and the "Ood", alien creatures who get controlled by Satan and attack everyone on board.
Indeed for the first half this is kind of a giant Aliens homage, or at least that's how I read it. Not that it's a bad thing though, on the contrary it's lots of fun and mostly terror seeing the characters have to maneuver through the ship to get out of the path of the Ood through ventilation shafts (the one guys shooting at the oncoming Ood made me think of it more than anything). Meanwhile the Doctor is at the core of the planet and decides, such as the Doctor will do, to go even further into the 'Pit', where... something may be.
The ultimate showdown doesn't have quite so much action as the Doctor figuring things out in his Doctor-y way (and as David Tenant does with abounding wonder with his "I GOT IT!" moments) as there are other things involved that I shouldn't go into too much (if you haven't seen it by now that is and are reading this). But as cheesy as the CGI for the beast is, it's the human beings that make up what works so well here. Rose is especially compelling, especially among the space crew who are all just trying to stay alive and not let any more people die. There's so much heart here that it's almost a shock this wasn't the season finale; a planet, after all, is sinking into a black hole with forms of Satan riding about, trying to reclaim (lost) power.
But such is the way of this show. Jolly lot!