It was anything but a faithful adaptation of the novel--and in this case, that was a very good thing. The book is a mess, and shouldn't have had Poirot in it, as it was really one of Christie's thrillers, but Gatiss did an excellent job of wrestling it into something that Poirot didn't seem out of place in.

Didn't he also do Halloween Party? That was a fairly faithful adaptation, just streamlined to fit the medium and amount of time.