Sommersby

Jack Sommersby (Richard Gere) goes off to fight in Civil War and when he doesn't return, his wife, Laurel (Jodie Foster), presumes he was killed in battle. Because he was an abusive man, she's not heartbroken and six years later, makes plans to marry another man so that her son can have a father.

When Jack suddenly turns up, he's a completely different man. He's now kind and attentive to both his wife and son. However, things don't add up. Even though he looks exactly the same, his shoe is different. He reads books that he would have never read before. He's generous with their neighbors, whereas before, he would never have shared anything. 

Laurel begins to suspect this man is not really her husband. 

Based on the 1982 French film The Return of Martin Guerre.