Who are Doctor Who villains Harmony Shoal – and where have we seen them before?

Another answer is, they’re “multi-nucleated organisms, functioning as an infinitely adaptable, independently functioning peripatetic central nervous system”. Thanks for clearing that up, Doctor…

But haven’t we seen them somewhere before?

Well, yes. They appeared in the previous episode of Doctor Who which – since it was a year ago today – you may not remember too clearly.

Last year's Christmas special, The Husbands of River Song, featured humanoids with the same diagonal scars across their faces and the same ability to open up their heads and store things in the blue goo inside their skulls. They went by the name Shoal of the Winter Harmony, hence Harmony Shoal for their fictitious research company in this year's episode.

It's interesting that the Doctor didn't make reference to their previous meeting, though. Despite the fact that the events of The Husbands of River Song took place in the year 5343 (so presumably 3,327 years after those of present-day Doctor Mysterio), in terms of the Doctor's personal timeline they actually happened just before he arrived in 2016 New York.

The Husbands of River Song was also the adventure in which the Doctor first met his current travelling companion Nardole, who he eventually rescued from the robot body of the tyrannical King Hydroflax (long story). Hydroflax had conquered Nardole's people along with numerous others including the Shoal of the Winter Harmony, who were hoping to pay tribute to him with a gift of the Halassi Androvar, the most valuable diamond in the universe (which River Song had been trying to steal from Hydroflax's brain – again, long story).

The bottom line is, there is a lot connecting The Husbands of River Song to The Return of Doctor Mysterio and therefore the Shoal of the Winter Harmony/Harmony Shoal to the Doctor. And while it's true that the events of the former took place thousands of years earlier than those of the latter, meaning the Shoal would be unaware of them, the Doctor would certainly remember.

Perhaps the Doctor is just suppressing the whole thing due to his grief over River. Nardole tells us he's been mourning her for 24 years, which you might think was log enough to get over it. But perhaps you've forgotten that the Doctor and River had their last date together on the planet Darillium, where each night lasts – that's right – 24 years...