Secret of the Blackbirds (1983)

Mr. Fortescue, owner of a big company, unexpectedly dies. The autopsy shows that he has been poisoned. Nobody actually loved him, and even his family members had their reasons to kill him. But who really did it? And who is responsable for killing his young servant and his wife within the next couple of days? Of course the only person who can answer this question is Ms. Marple. And of course, the murderer is the person you least expected it to be.