In the sixth episode of the "Mysteries of Paris" series, titled "Secrets Beyond the Grave," viewers are taken on a journey through the dark and mysterious side of the city's history. The episode explores the forgotten and hidden secrets buried beneath the streets of Paris, including catacombs, underground tunnels, and secret societies. From uncovering the remains of ancient civilizations to investigating famous unsolved murders, this episode delves into the secrets that lie beyond the grave in the City of Light.