Using Agatha Christie's name for this film was misappropriation. The secret passage was an overused one-trick pony, while the dead body was merely an after thought. The music background must have been mistakenly dubbed in. The detective in plaid suit and carnation suggestively clownish when he should have tied the plot and action together. The servants and the little girl were time wasters to overdrawn dramatics. Glynis Johns in a pretty green dress was the lead...uh huh.
In conclusion, no kudos to writer or director; maybe a firing squad? My one star is given as a personal wish that this confused, boring film disappears down a rabbit hole.