Christopher "Kit" Francot is the younger twin. His "twin-sense" tells him that something has happened to his elder brother, Evelyn, Lord Denville, and so he slips home to see what's the matter. It turns out Evelyn has disappeared on the eve of his presentation to his nearly-betrothed's grandmother - if Evelyn doesn't appear, granny won't give her consent and Cressida Stavely's much-needed dowry will slip away (neither Evelyn nor Lady Denville have much head for money...at all). Lady Fancot pursuades Kit to masquerade as his brother for "just one night" setting off a tangled web of mistaken identity - the False Colours.
Very much a comedy-of-manners - one twin masquerading as the other, Lady Denville is a pretty spendthrift, her most ardent suitor and friend is a lovable hedonist, and the boring family relatives drone on about "economizing" and the "precarious" health of their son (who is on a repairing lease).
The slang flies fast and furious in this novel. I consider myself fairly well-versed but even I had to look up a few. So Heyer did her homework.
In previous Heyer novels I complained a bit that the hero and heroine only come together in the last chapter. In this one the couple acknowledges their feelings about halfway through then spends the rest of the novel getting out of the sticky twin masquerade.
Lots of fun.