Hercule Poirot is determined to solve the deaths of a husband and wife from years ago. Was it a double murder?

So Poirot went to the cliff-top. Here, many years earlier, there had been a tragic accident. This was followed a few weeks later by the grisly discovery of two more bodies - the husband and wife in question - shot dead. But who had killed whom? Was it a suicide pact? A crime of passion? Or cold-blooded murder?

Poirot delves into the past and discovers that "Old sins leave long shadows". When he and Ariadne Oliver look into the events, they are left asking whether humans can remember as well as elephants.