The Devil's Maze.

While visiting her friend, Barbara Carlton, Frances Mildmay spends the day fox-hunting and ends up fogbound in the 'Devil's Maze' with Barbara's husband, James. When the pair lodge overnight at an inn, the innkeeper convinces James to seduce Frances. Unbeknownst to them, another family friend, Derek Riffington, is also staying at the inn and becomes aware of the liaison. But, by the time James discovers Frances is pregnant, Derek has gone missing in the Far East - leading James to suggest that, when the child is born, she should accuse Derek of having been the father.