While the opening episode of Doctor Who’s tenth series was all about Pearl Mackie’s new companion Bill, eagle-eared fans will have a noticed a sweet little callback to previous co-lead Jenna Coleman’s Clara Oswald towards the end of the story.

And yes, we did say eagle-eared – because the reference was concealed in a subtle musical cue, cropping up at the end of the episode as Bill berated the Doctor (Peter Capaldi) for trying to wipe her memories of their adventure.

"Ok, do what you’ve gotta do but imagine, just imagine, how it would feel if someone did this to you,” Bill snaps at the shamefaced Doctor, little knowing that her tutor knows EXACTLY how she feels after having his memories of Clara removed last series – and then just to emphasise the point, a brief snippet of composer Murray Gold’s “Clara” theme plays over the scene.

The musical moment was a brilliantly subtle way to invoke Clara’s memory in the new series, and given that this particular theme might be more familiar to viewers than most (after the Doctor performed it at the end of series 9 on the electric guitar) we’re sure there are plenty of fans who noticed it. 

Still, if you didn’t you shouldn’t worry – you can just keep your own eagle ear out for it on your next rewatch.

Doctor Who returns to BBC1 next Saturday on BBC1 at 7:20pm