The Two Faces of Evil - S1.E12

The Lewis family, Martin and Janet and their son David, is driving off on holiday. As they go through a severe rainstorm, they almost fail to see a man in the road, missing him by inches. Martin stops and offers the sober-looking stranger a lift. Back on the road again, the stranger suddenly attacks Martin, causing a terrible crash. Janet and David wake up in hospital and find that Martin has also survived, badly scarred and changed... As time goes by, Janet starts to wonder whether the man who is now her husband is Martin at all. Could it be the stranger?