This is one of those Agatha Christie stories that relies on a great ability to suspend disbelief, because it's being filmed. On paper, the pivotal part of the story is easy to disguise, but on film you need a very good make-up team to fool the viewer.
I don't think I'm particularly putting a spoiler in here, because one look at the cast list reveals what the secret is.
It's a good story, but not one that's difficult to see through. The cast and settings help the story through though, and Joely Richardson and Mary Tamm are good to see propping up the cast.
A good story - but better on paper.