I think this might be both the Doctor and Rose's best episodes in all of Series 2. I probably shouldn't be surprised that the episode where I like them the most is also the episode where they never share the screen until the very end of the episode.

I'm not going to spend long on the plot. It's good and it moves but I haven't much interesting to say about it. The Doctor explores the Satan Pit whilst Rose and most of the remaining crew of Sanctuary Base 6 are trying to survive the possessed Ood. Through Rose's leadership, their individual skills and some teamwork the remaining crew make it to the ship and fly off the base. Unfortunately Toby is still possessed by the Beast, who is using Toby to escape his prison. Fortunately the Doctor works out that the Beast was imprisoned on this planet for a reason. The energy that is allowing the planet to stay in orbit around a black hole, is also keeping the Beast imprisoned. So if the Beast were to ever escape his prison, the energy would stop, causing both the planet and the Beast with it to plummet into the black hole. The Doctor causes this to happen, finds the TARDIS and rescues everyone whilst Rose kills the possessed Toby.

Rose is really good here. Whilst everyone has been affected by the Beast taunting them, and are currently panicking, Rose keeps a level head. She's been with the Doctor long enough to know what to do in these situations. She takes stock of what she has available to her. She doesn't know the base or it's contents, but she knows she has a capable crew who do. She also identifies which problem most urgently needs solving. In this case it's that the crew are trapped (Zachary the acting captain is trapped at the bridge. Everyone else by the drill) and the murderously possessed Ood are breaking through the doors. So she motivates her team to solving the problem at hand. Danny (who is in charge of the Ood) realises that he has a way to turn off the Ood's telepathic field, effectively incapacitating them (at least temporarily). However they need to get to Ood habitation for this to work, and that currently seems impossible. Mr Jefferson (in charge of security) realises how to get there, through the maintenance tunnels. The problem with that is that there's no air in those tunnels. Zachary (acting captain) realises that on the bridge he can manipulate the atmosphere inside the base and manually move the air around so that everyone else can get to Ood habitation. Toby (the archaelogist in charge of studying the ancient civilisation that have imprisoned the Beast) does... umm... well he pretends not to be possessed and looks at Rose's bum. Ok so they're not all winners. But by working together they are able to successfully incapacitate the Ood and retake control of the base, albeit with the loss of Mr Jefferson, who sacrifices himself to buy everyone else enough time to escape.

It's a wonderful sequence that gives (almost) everyone something of value to do and demonstrate the worth of every person. Take Zachary who has been promoted to acting captain due to the death of the original captain for example. He feels unworthy of the role, as well as useless being trapped in the bridge where he feels he can't help his crew, only able to push some buttons. Shaun Parkes is great in these 2 episodes btw. It's rivetting watching him just push some buttons, and seeing him demonstrate his worth as the captain he was never supposed to become. He also sells the moment a little later where Rose refuses to leave with them on their ship, so he orders his crew to hold her whilst he drugs her unconscious and carries her onto the ship. This scene could be uncomfortable, seeing 3 older men restrain and render a young woman unconscious against her will. But Parkes really sells this as an act of kindness, and you feel it when he explains that he isn't prepared to lose anyone else on this base.

The other side of the story we have the Doctor and Ida down in a chasm. I quickly want to shoutout Claire Rushbrook's performance as Ida. She's also great in these episodes. Her reckless curiousity feels natural and charming, when it could've easily seemed annoying. I also really feel for when she's afraid of dying alone.

But this episode really plays to David Tennant's strengths. More than maybe any other Doctor (certainly of the modern era) Tennant is great at tackling long speeches. This pays off here because although he has Ida to talk to at the start of the episode. For a decent chunk of it the Doctor is basically just talking to himself or a non-verbal, satanic monster. But if you give David Tennant something worth saying, you can just leave him to monologue and it's thrilling to watch. I can't imagine any other Doctor doing such a good job making all the exposition at the end feel as natural or compelling as he does here.

I don't think the question about whether the Beast is literally Satan or not is that interesting. I think it's clearly supposed to be an ancient monster where the idea has persisted and formed the basis for many satanic figures. But that it isn't literally Satan. It's a fun idea worth some dialogue though, but ultimately I don't think the episode is too interested in answering this question. At some point the Doctor realises he doesn't need to know what he's fighting, he just needs to know how to beat it.

I do like that this idea has the Doctor talking about belief. What his beliefs are and questioning them. I think there's interesting stuff in there. I particularly like his line about how he would have no trouble believing the Beast came from beyond time, but he cannot bring himself to believe the Beasts claim that he comes from before time. That doesn't fit with his beliefs, but he can't justify why it can't be, besides that it breaks the rules he's given himself to explain his beliefs.

But all of this belief talk is partly there so that the Doctor can give his little speech before choosing to send the Beast into the black hole. The Doctor saying "But I’ve seen a lot of this universe. I’ve seen fake gods and bad gods and demi-gods and would-be gods, and out of all that, out of that whole pantheon, if I believe in one thing, just one thing, I believe in her." actually works for me here. It helps that the episode has done a good job showing why Rose is someone worth believing in. This all adds up to me actually liking the Doctor/Rose dynamic for the first time this season. It's still a problem that it took 9 episodes to make this happen. But's it's better than nothing.