When wealthy businessman Rex Fortescue collapses and dies over tea in his office, the police are surprised to find his pocket full of rye. When they figure out how he was poisoned, they realize it must have been over breakfast not tea. That leaves his family in the frame.
His much younger new wife is having an affair, so she is the obvious suspect—that is, until she collapses over tea. Then Gladys, the house maid, is found with the laundry, strangled and with a clothespin on her nose.
Miss Marple arrives on the scene after she reads of Gladys’s death, having trained Gladys to be a maid. She is the one who makes the connection between the deaths and the old nursery rhyme. But then, what about the blackbirds? Could this have anything to do with the Blackbird Mine, over which Fortescue reputedly cheated a partner?
This is one of Christie’s more ingenious mysteries. It hangs together without seeming absurd even though the murders seem deranged. I also thought the ending was quite effective.
What? Me Read? 
This has always been one of my favourites, I think because it’s a real example of the murderer being the most unlikely person and yet it’s still convincing.
It’s a good one.