Wow -- do we really have nothing but ten-star reviews, broken only by one three-star? Neither is accurate, nor useful, so here we go:
The good news is that the writing quality has returned. Make no mistake: it's still largely a high-concept soap opera, with melodrama abounding -- and you'd better not ask how it is that a book belonging to Edith's employer is found in the house ... unless you're the sort of person who would bring a book on German grammar to a party, when your main reason for being there is to see the Lord's daughter. Also, the whole Baxter-Barrows drama feels a bit forced, and the Bates business is being dragged out to no real advantage, in typical Downton Abbey fashion.
But never mind all that: Robert James-Collier is fine (once again!) as both villain and hero, Maggie Smith gets some lines she can really deliver with incomparable relish (as does Penelope Wilton, to a lesser extent), and there are two very good party scenes, where relationships are changed and interesting things are said. If the episode concludes with an event clearly designed to allow certain interactions to happen -- well, they had to happen somehow, and it mostly works if we don't question what happens with Gregson's book too closely.
In sum, a good solid episode: neither a favorite nor an embarrassment, with some delightfully piquant dialog -- a distinct improvement over the here's-our-plot scripting that dogged much of London Season. Speaking of dogs: Isis gets three scenes in this episode, and is specifically mentioned twice (though off-camera). And if I didn't mention how fabulous all the clothing and hair is -- Elizabeth McGovern never looked better than the early scene of her coming down the stairs. Beautifully shot and she looks born to dress that way. Equally, Allen Leech is the very picture of a dashing young man in his dinner outfit. That's just the two actors who stood out to me in this episode.