For all the awful Mary Sue river song garbage that follows, one could weep.
But in this first story two-parter the character serves its purpose and doesn't detract from some classic horror beats.

Terrible. Even by the standards of the current pantomime / SJW dross that Who has decayed into this is abominable. Interminable. And perhaps worst of all, so terribly unnecessary. This OCD fan fiction hacktivist compulsion to explain, and explain, and EXPLAIN until there is no mystery, no wonder and no classical Who left... Will kill the show stone dead if it doesn't stop.

The actual plot such as it is doesn't rise above the current pantomime level, and the performances are not over the top, but Capaldi has never been a good actor - someone who broke in via Lair of the White Worm was never going to be, was he? And Kingston is as rubbery, unconvincing and odiously smug as ever. Probably just can't believe her luck that there's enough of a fan ghetto to support her declining years.

A sequel (but in terms of time frame a prequel) to the Fourth Doctor television story Terror of the Zygons.

It isn't very well written, and it is strangely unengaging at the best of times. The introduction of the Zygon sting which was edited out of the television story.

The story meanders around and wraps itself up but the payoff is dim and the lack of any real point to any of it, not to mention the monstrous (pun intended) incongruity of the set piece, leaves it as a bit of a wreck.

A clever conceit cleverly confected.

There are a few particularly nice little comments giving insights into the fictional universe of Who, and a nice explanation for what it is we're actually watching when we watch Who.