The Third Walker (1978)

Colleen Dewhurst portrays Kate, a woman who begins to suspect one of her twin sons, James, is not her biological child when a boy, Etienne, who is identical to her other son Andrew enrolls in the same school. Further investigation determines that Etienne was born in the same hospital on the same day as her children. Kate pursues it in court and tests confirm Etienne and Andrew are twins. Kate becomes obsessed with getting Etienne back, while Marie, who has been raising him as her own, does not want to give him up and is not interested in getting custody of James, who is her biological son. She cannot understand how Kate could simply divorce herself from the child she has raised from birth. A judge orders that the boys be switched to live with their biological families, so Marie disappears with Etienne. This haunts all involved for the next twenty years. Kate drives her husband (William Shatner) away with her obsession. She continues to raise James, but dreams of reuniting with Etienne. James was terrified when he thought he might have to go live with Marie and is tortured by the fact that the woman he's loved as his mother loved him less when she found out he was not her real son and would have given him to a stranger. Andrew loves James as a brother and they are best friends, but Andrew has also lived with curiosity about Etienne. When their father resurfaces after many years only to be killed in a bizarre accident, Etienne comes back into their lives. James feels left out and unwanted and must decide where he fits in. Scenes from the past and present are strangely edited together.