His Dark Materials makes its biggest change yet from Philip Pullman’s books

**Warning – we’re dealing with serious episode five spoilers from hereon out, so read on at your own risk**

Specifically, we’re talking about leading book character Will Parry, as played by Amir Wilson in the TV series. In Pullman’s text Will first crops up in the second book (The Subtle Knife, bracketed title fans) as a troubled young man from our world who accidentally passes through to another, meeting Lyra (Dafne Keen) and getting swept up in an epic battle between good and evil.

• His Dark Materials episode 5 review: Where there’s a Will, there’s a way

In the TV series, though, he’s been introduced early – quite early, in fact – as Lord Boreal’s (Ariyon Bakare) investigations into other worlds begin to bear fruit. Since episode two, Boreal has been popping back and forth between “Lyra’s world” – aka the main universe of the story filled with armoured bears, daemons and the like – and “our world” to try and track down Jordan academic Stanislaus Grumman, aka ex-soldier Major John Parry.

During this episode, his mission leads him to place a watch on Parry’s son, Will (Wilson) and his mentally unwell mother (Nina Sosanya), causing her some distress and making Will suspicious about his father’s long-ago disappearance. At the same time, we learn of an ancient prophecy that appears to link both Lyra and Will with the challenges to come, teasing future events in another addition to the story (in the book, any prophecies mainly concerned Lyra).

At the opening of The Subtle Knife, Boreal’s investigations and his men’s snooping were told in flashback as Will ran away from home, but in this version of the story we’re seeing it unfold in real time, as well as adding some extra story for Will (including a boxing scene where we see how he’s struggled with his mum’s illness and being bullied at school) - and according to the people who made the show, this sort of added backstory is part and parcel of their adaptation process.

“I've never read a trilogy where this doesn't happen, Lord of the Rings is a classic example of this – where the novelist builds what they've done in book one, then they back-fill moving forward in books two and three,” His Dark Materials executive producer Jane Tranter previously told RadioTimes.com of changes made to the source material.

“I don’t think novelists are like, 'Well, I've worked all that out and I'm just not going to tell anyone that that's what this small gesture means in book one until book three'– they sit writing book two and three with book one in their mind, and then they build and they elaborate on it.

“And so by the time you reach the end of the trilogy you understand more what was going on in book one. I think in an adaptation, and we did the same on Discovery of Witches, you adapt book one, knowing what you know, having got to the end of the journey in book three.

“It doesn't mean that plot-wise things change, but character-wise or in terms of things that you shade in or bring out more prominently – they’re all there to be had.”

In other words, because we’re seeing what Will, Boreal and his mum were up to off-screen during the events of first book Northern Lights (which in Pullman’s text only takes place in one world and specifically follows Lyra), technically we could call this an addition rather than a “change.”

And of course, in all likelihood it’ll still be season two when Will and Lyra meet properly (unless the timeline is getting seriously accelerated) and Will’s early appearance shouldn’t really make much of a dent on Lyra’s upcoming battles in the north.

But despite this, and despite Wilson’s terrific performance as the angry, frustrated Will, it’s not hard to imagine some of Pullman’s readers decrying this latest change to his text. Fingers crossed the on-screen Will is compelling enough to convince them it was the right call.

His Dark Materials continues on BBC One at 8pm on Sundays