[This is a review of Doctor Who season 9, episode 3. There will be SPOILERS.]

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Doctor Who has done horror before, in fact, last season's 'Listen' was a particularly well-done excursion down one of the Doctor's darker avenues. It was creepy and frightening in all the right ways, and while it hinted at the supernatural and the unknown, there wasn't any actual confirmation the entity they were dealing with was paranormal in any way. And that is how things work on this series; the show is at least partially about exploration and discovery, which means there is typically an answer to whatever the Doctor and his companion encounter. An answer that is usually found in the corporeal world – or at least as corporeal as things get on a series like this.

'Under the Lake,' then, takes the idea of the non-corporeal realm making contact with the most curious being in the entire universe. And the results are pretty much what you'd expect. Peter Capaldi's Doctor has been marked by his tendency towards crankiness and darkness – which actually makes him an ideal Doctor to tackle a potential story about ghosts – but when he comes face to face (literally) with a pair of apparitions on the BBC's live-action version Sea Lab 2021, his reaction is in keeping with all iterations of the character. That is to say, he's not frightened; he's curious, and then he gets really, really excited.

There is a certain kind of joy in the Doctor's desire to experience things first hand – especially things that are frightening by definition. That joy is partially responsible for connecting the character to the audience, as those watching are generally doing so to experience something fantastic and amazing, and so when the Doctor comes across something like, say, a real ghost, his reaction mirrors that of the audience. It's in these moments that the Doctor actually becomes the proxy for those watching, and his companion – whomever it may be at that particular moment – is really just there to help move portions of the plot along.

The cast of Doctor Who Season 9 Episode 3

That is a risk that's taken in certain episodes of the series, where the Doctor's companion is marginalized more than he or she usually is. For all the potential excitement of discovering ghosts in an underwater military facility, 'Under the Lake' doesn't quite manage to spread that elation around as evenly as it could have. Some of the problem is that Capaldi's performance – especially when he's gushing about having just met a ghost – is so big that it tends to suck all the air out of the room. And that can be an issue when the story takes place where oxygen is actually at a premium.

Thankfully, the episode knows how utilize the Doctor's palpable enthusiasm as a means of including the other players – meaning the crew of the underwater facility, not just Clara, whose appetite for adventure seems to have been the impetus for the pair's trip to a waterlogged lake bottom. At some point during the course of the episode that means the Doctor takes on the role of teacher – which actually suits Capaldi-Who quite well, as it allows him to be scold-y without coming off as though he's a complete and utter misanthrope. Instead, the Doctor is there to impart knowledge upon his would-be charges, especially O'Donnell (Morven Christie), who takes a real liking to him, and the effect gives a teacherly quality that is in keeping with the Doctor's eagerness in discovering more about the facility's supposedly supernatural guests.

This leads to a fun sequence in which Clara and the crew lead the ghosts through the circular corridors of the facility, leading them to a Faraday cage. The fact that the apparitions can't penetrate the cage's walls, leads the Doctor to come to the conclusion the specters are really a signal heading off-planet, presumably to where the spaceship the crew had unwittingly discovered amongst the ruins of a flooded village originated. The revelation is not an unexpected one, given the series, but the dime on which the episode turns is rather impressive, as it doesn't work to solve the mystery, but to deepen it and to justify the story taking two hours to tell, rather than just one.

Ghosts in Doctor Who Season 9 Episode 3

While a self-contained episode would have been preferable, after the previous two hours were spent dealing with Daleks in a rather choppy extended season opener, writer Toby Whithouse successfully expands the mystery at the episode's core in such a way a continuation feels more like something the story earned, rather than an obligation to perfunctorily expand the scope of a tale that doesn't necessarily need it. And since the episode begins as an engaging ghost story with a pair of disturbing specters driving the plot – those empty eye sockets and equally empty skulls, combined with mouths breathlessly repeating a phrase over and over again is the sort of nightmare-inducing image Doctor Who excels at when it wants to – only to turn into an intergalactic mystery by way of The Abyss, 'Under the Lake' does, in some sense, feel like a self-contained episode.

That might only be because the continuation of the story will have such a specific and different goal in mind – given that the Doctor appears to have died in his journey to a time before the village in question was flooded – but it works to make this adventure more compelling and propulsive, and less reliant on the next hour to justify this one. Sometimes Doctor Who tries its hand at genre, only to come up with something that walks the line between two specific kinds, and here it succeeds at making the initial effort as strong as the abrupt turn towards illumination. The result, then, is two journeys that appear worth taking.

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Doctor Who continues next Saturday with 'Before the Flood' @9pm on BBC America. Check out a preview below: