"Under the Lake"/"Before the Flood"

The basis (under siege) for this story is rooted in several well-worn tropes of science-fiction in general and Doctor Who in particular - indeed, beats are taken from the Tennant classic "The Impossible Planet"/"The Satan Pit", most notably the death by airlock with the body floating outside the base window. However, in the very capable hands of Toby Whithouse, we are constantly left scared and surprised at every turn in a story that gives us terrifying underwater hauntings, time-bending paradox crunching, Cold-War era dummy towns, Arthurian legend, and love, both unrequited and requited. The explanation for the ghosts and their motivation is revealed only piecemeal, which makes re-watching the story all the more rewarding and, at no point does it alienate anyone who had given the story their proper attention – the untranslatable characters on the spaceship wall are the seeds of a ghost factory, the TARDIS is wary because the Doctor is already there in the hibernaculum. The Fisher King is a far more malevolent figure than his Arthurian namesake, yet the roots in the myth are present and correct – both require those who venture upon his abode to assist in his return...

The production is flawless in realising Whithouse's vision – the ghosts are designed, realised and filmed to live in the nightmares of many in years to come and Daniel O'Hara makes a stunning début as director crafting each scene with care, an obvious highlight being the ghost of Moran dragging an axe just a few paces behind Cass. The design work is fantastic, from the sets for the Drum, to the dummy town to the memorably crustaceoid (if that's a word) Fisher King.

The characters are somewhat atypical for the base-under-siege formula and the actors do stunning work. Morven Christie manages to be completely convincing both as a hard-as-nails NCO and as an utterly adorable Doctor-fangirl. Whithouse never makes Cass a mere token character and the gutsy performance that Sophie Stone gives in the role only adds to the effectiveness of the character. We also have a nice turn by Paul Kaye as the Tivolian, Prentis. It must be said that it is a pity that Colin McFarlane has so little to do in his non-ghost form, but it is no surprise that this very charismatic actor is the scariest of the ghosts. The imposing size of the Fisher King (courtesy of Neil Fingleton) is matched with a memorable turn by Peter Serafinowicz, one of the most versatile voices in the business.

The Doctor's job is a bit more complicated than in should be. How can a time-traveller defy his own ghost? Very well indeed, if the Doctor is as brilliantly written and performed as he is here. Capaldi portrays the Doctor as being stoic in the face on inevitability, totally in control of the end of the moment, if not it's beginning, yet needing cue-cards to help him console those he is about to help. He is also given the most lengthy piece of fourth-wall breaking in the programme's history, which he pulls off with aplomb. With the characters separated, Jenna is left to be the lead in her segments, which she does magnificently.

So, we are left with the Bootstrap Paradox and a question that is never answered. But don't some of the best stories end with a question mark?

NEXT: "The Girl Who Died"/"The Woman Who Lived"