An ''Unsolved Mysteries'' spin-off -- ''Final Appeal'' will debut this fall on NBC

A former police officer is doing time in a West Virginia prison for murdering his girlfriend — but witnesses insist the woman is still alive and in hiding. A Los Angeles man is in jail for killing his wife and two children — but nobody can explain how he supposedly shot himself in the back while committing the crime. ”There are lots of bizarre cases out there,” says Unsolved Mysteries host Robert Stack. So many, in fact, that this fall NBC will launch Final Appeal: From the Files of ”Unsolved Mysteries,” a spin-off series that will investigate closed criminal cases in which the accused insist they have been unjustly convicted. ”We’ve done this sort of thing before as segments on Unsolved Mysteries,” explains the 73-year-old Stack. ”We present both sides and let the viewers decide for themselves what actually happened.”

The half-hour series may not reverse any convictions, but it may help solve at least one mystery for NBC: how to counter-program against the big Friday-night lineups of ABC (Family Matters, Step By Step) and CBS (Golden Palace, successor to NBC’s Golden Girls, and Major Dad). How does Stack think the spin-off will stack up? ”To tell the truth, I never thought Unsolved Mysteries would be a hit. I’m more of a machine-gun fire and screeching-car-chases guy. But I’ve learned to expect the unexpected. Life to me is one big unsolved mystery.”