Go Back for Murder by Agatha Christie

Imprisoned for poisoning her husband fifteen years earlier Caroline Crale finds herself at death's door and writes a letter expressing her undeniable innocence to her daughter Carla. Carla aided by Justin Fogg - a former admirer of Caroline - persuades those present on the day of her father's death to return to the scene of the crime. When the witnesses reassemble Fogg discovers the identity of the true murderer and Carla discovers her true feelings for Fogg.