Last night’s Doctor Who special, ‘The Waters of Mars’, was such a tonal shift from Easter’s execrable ‘The Planet of The Dead’ that I had to double check their credits. How could Russell T Davies have penned both the former’s tense treatise on the laws of Time and the latter’s Punch & Judy exercise in limp sci-fi generics?

Maybe Davies felt he had to approach ’Mars’ as serious preparation for the return of The Master, so chose to blow off some kiss-me-quick steam on ‘The Dead’, in which all The Doctor had to do was turn up & grin. Lacking a companion and a character arc, he was reduced to an ensemble player in a frantic but fun-free farce. In ‘Mars’ he watches people die, walks away, then crosses the line. He flips as far into the dark side as we’ve ever seen him go, buckling under the pressure of being the last of his kind, and you see how close to The Master he truly is.

There are some hiccups along the way, obviously. The story opens with The Doctor arriving on Mars and taking a long walk to Bowie Base One, the first human attempt to colonise another planet. When he realises he’s arrived on the day the legendary colonists are all destined to die, he feels he has no choice. This is a fixed moment in Time he cannot change. Or can he? The ‘monster’, in this instance, is ingenious. It’s water. Trapped beneath the polar ice of Mars, released by the colony, and eager to feast on the oceans of Earth. A single drop can possess a human, and it has the power to seep through any seal or steel, so soon everyone’s running for their lives and the Base’s Captain has to make a choice: accept the consolation of an inspirational legacy, or rage against the dying of the light.

It’s genuinely scary in a way that’ll probably haunt younger viewers for years to come, and the dialogue is some of RTD’s best since ‘Midnight’, but Lindsay Duncan embues the Captain’s role with a dignity that unfortunately makes her final act seem all the more pointless. After the Doctor’s fit of temporal megalomania, her attempt to wrest her destiny from him comes across as either paradoxically weak or idiotically petulant.

More minor faults include the introduction of an annoying robot with a goofy catchprase for ill-fitting comedy value, an oddly abrasive relationship between the Captain and her XO – played by ex-’Neighbours’ beefcake ™ Peter O’Brien – that’s never fully explained, and an irresistible but nevertheless face-slappingly obvious backlit hero shot for when The Doctor finally returns to save the day.

Luckily none of this detracts from the episode’s main appeal which is, as ever, David Tennant’s Doctor, here fighting a crazed impulse to subjugate Time to his own grief-devastated will.

 ”The laws of time are mine, AND THEY WILL OBEY ME!!” is not a line you’d find in any other Doctor Who episode, and that’s the clincher for me. ‘The Waters of Mars’ shows a side of The Doctor that’s uniquely skewed while still being a logical development of the character over the last season. Disconnected from his people, and now disconnected from his companions, he is on the verge of madness toward the end of this episode and it’s almost unbelievable, but you still understand it.

After ‘The Planet of The Dead’ I wasn’t looking forward to ‘The Waters of Mars’, but now I can’t wait till ‘The End of Time’.