The most striking thing about Death in Fancy Dress for me was the contrast between my expectations (based on the title, the cover, and the first chapter) and the reality. It appears to be a light-hearted romp, with quick-witted Jeremy Feltham rooting out evil blackmailers and winning the hand of his girl. That is very much not what the rest of the story is about. I don’t mean that the book subverts those tropes – instead, it drops them almost completely. Its tone is much more serious than I expected, too.
As Dorothy L. Sayers put it in her review, “So often in a detective story trivial irregularities like blackmail and murder seem scarcely to ruffle the placid current of domestic affairs… Here, the atmosphere of suspense and uneasiness really does pervade the household.”
This is what the book is the most effective at – a slow build of a tough dilemma, peaking with the murder, which then spills those problems out into open view and forces the characters to try and contain them.
I would note that in terms of page count, the blackmail causes more ruffling than the murder. By “the blackmail”, I don’t mean the massive blackmailing operation that draws the narrator and friend into the plot, I mean the blackmail involving Hilary in particular. What the plot mostly all about is who Hilary is going to end up marrying: hidden-fires civil servant Arthur, adventurous Jeremy, and rascally cousin Ralph all declare their intention to fight for possession of Hilary. I say “possession” because none of these men seem interested in what Hilary thinks. If I cared about Hilary, I wouldn’t want her to end up with any of these guys.

Which brings us to the characters. Unfortunately, Hilary, despite being central to the plot, shows little of her character except desperation and irresponsibility. To be fair, she’s in a tough spot. But there’s nothing to show why all the three men might want her, or why she might be a good match with any of them. The particular problem of who she might marry held little interest for me.
Actually, the most interesting characters are the older ones, particularly the older women. Eleanor Nunn is the aunt of narrator Tony, and has an interesting backstory as “the woman behind the man” for a famous politician. Meriel Ross is the widowed sister of Eleanor’s second husband James, who is a wealthy self-made man. Meriel the “merry widow” has an unique outlook on life and particularly on the customs of the upper classes, which seem strange to her. She also gets a lot of witty lines.
Outside these two, the characters fade into the background to some extent, though there is also the extremely hateable villain Ralph Felton.

The introduction of Ralph and the blackmail of Eleanor and Hilary poses a problem for the plot. Tony, Jeremy, and Arthur all immediately focus on that, and at no point do they actually investigate the Spider (the head of the blackmailing ring) during the book. This is similar to how Jeremy declares he will break Arthur’s engagement to Hilary, but never actually does anything to that end. There’s a lot set up that never really pays off. At the other extreme are things like the costume party and the murder, which happen very abruptly quite a decent chunk into the book. One reason an author might leave it late to include their murder is to build up the reader’s connection to the characters before that point, but I didn’t feel particularly invested in the guilt or innocence of any of them. The investigation is packed into the final third of the book, and is pretty barebones, including one blatantly telegraphed incident where the narrator accidentally uncovers the key clue.
However, despite that, the clueing and misdirection are actually quite good and have some clever aspects. I’m not sure the distinguishing characteristics that mark out the true solution from the false are entirely clear, though.

One thing I did enjoy, particularly at the start of the book, was the writing. Anthony Gilbert (an alias of Lucy Beatrice Malleson) is pretty adept at observation and turns of phrase. Her skills allow her to coast by and create something readable, even though structurally the book is a mess which never finds a clear focus. Characters spend a lot of time telling backstory and talking to each other, and much less time actually doing anything. “Villains act, heroes react”, even when the heroes spend an awful lot of time threatening to act.

The British Library Crime Classics edition also contains two entertaining short stories, “Horseshoes for Luck” and “The Cockroach and the Tortoise”, which I enjoyed more than the main book. Both involve Inspector Field recounting his past cases to his fellow-drinkers in a pub. They’re simple stories, but Field’s narration, and the occasional contributions from the other drinkers, are what make them fun to read.

Other opinions:

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