Elephants Can Remember (Hercule Poirot #40) by Agatha Christie

Hercule Poirot is determined to solve an old husband and wife double murder that is still an open verdict! Hercule Poirot stood on the cliff-top. Here, many years earlier, there had been a tragic accident. This was followed by the grisly discovery of two more bodies -- a husband and wife -- shot dead. But who had killed whom? Was it a suicide pact? A crime of passion? Or cold-blooded murder? Poirot delves back into the past and discovers that 'old sin can leave long shadows'.

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