First, a spoiler warning. This article will contain spoilers for something first transmitted on British television over 50 years ago. Why mention it? Because I would normally be desperate to avoid them.

I’ve been a fan of Doctor Who since Tom Baker was in the TARDIS. I remember being stunned the first time I found out that 100 or so episodes were missing, junked by the BBC in the 70s. I’ve spent decades fervently hoping they will turn up. But I also decided to avoid knowing anything about them beyond their names: I don’t look at telesnaps, or watch reconstructions, or listen to the audio versions. I even steer clear of the most basic synopsis.

This means that, very occasionally, I get to watch brand new Doctor Who. It happened when The Reign of Terror and The Underwater Menace came out on DVD, and most jaw-droppingly when The Enemy of the World was rediscovered in 2013. Now, it has happened again: BBC Worldwide has brought Power of the Daleks back to life as an animation. All I knew in advance was that the Daleks most likely got defeated. But essentially, I had six new episodes ahead of me.

It was much more tense than I anticipated. There’s huge interest in the story’s first episode in particular: it was Patrick Troughton’s debut as the Doctor, and it followed on directly from the first time Doctor Who ever changed face. I assumed he would pick himself up then head off on a rip-roaring adventure with the Daleks (an enemy chosen to assure the audience that this was still the same show).

Instead there’s a lengthy adversarial TARDIS scene, where Troughton does nothing to reassure us he is the same man that the first Doctor Who, William Hartnell, was. He keeps referring to The Doctor in the third person and past tense, leaving the viewer (and companions Ben and Polly) guessing. And the long pauses, with just the hum of the TARDIS audible, are excruciating.

Despite being the big draw, the Daleks only feature for a fleeting moment in the first episode. Instead, it is all about setting up the environment in which the adventure will take place: a colony on Vulcan, your classic futuristic-base-under-siege.

A scientist named Lesterson is investigating a mysterious alien capsule found abandoned on the planet. Inside it are three inert Daleks. You don’t need to have avoided spoilers to realise that scientists attaching electrodes to dormant Daleks is a bad idea, but the slow awakening of the Dalek being experimented on in episode two is as chilling as it is inevitable.

In some stories, Daleks can be reduced to little more than shouty stormtroopers, but they were excellently written by David Whitaker here. They are cunning, manipulative and duplicitous – and the sequence where it becomes clear they have been secretly manufacturing a Dalek army is brilliantly realised in the new animation (the original used cardboard cut-out Daleks to boost the ranks.)

Power of the Daleks has always been well regarded by fans. Despite being entirely missing, it was voted the 19th best story of all time in Doctor Who Magazine’s 50th anniversary poll. In part, this is because of the slow inexorable march over six episodes towards the grim conclusion. The leaders of the colony believe the Daleks will bring them wealth, fame or power. Only the Doctor (and of course the audience) have seen them in action and know what to expect.

Lesterson’s descent into hysterical remorse as he realises the full horror of his Dalek revival was unexpected to me. All that remains of Robert James’s original performance is the audio, but the scientist sounds utterly broken by the time he faces death.

Power of the Daleks’ ending is very bleak indeed. There’s a panning shot of corpses littering the colony floor, exterminated. It’s amazing to remember that in 1966, when it aired, the show was unambiguously marketed at children, yet could still contain such an explicit on-screen massacre.

The animation has its limitations. While the sets and backgrounds look gorgeous and the faces are expressive, the technique doesn’t cope well when characters move around; at times it is rather more Oliver Postgate than Pixar. It’s also been made in 16:9, in a move that has shocked some fans. Animator Charles Norton argued that most people will watch it on widescreen TVs, so why not do it in that format? But if there’s one fandom adept at wrangling a 4:3 picture out of their DVD players it’s Whovians. Side-by-side comparisons of the programme with surviving telesnaps show the extent to which some shots have had to be reimagined.

Twelve televised Doctor Who stories from the 1960s remain that I know nothing about beyond the titles. Only time will tell whether this release will be enough of a hit for the BBC to animate another – or if there are any more missing episodes still lurking on a dusty shelf somewhere.

• Power of the Daleks is available to stream or download from the BBC Store, and will be released on DVD on 21 November.