An early (late '70s) HP minicomputer text adventure game, this one begins with the player dropped off by a taxi outside the locked gate of an estate containing an eponymous mysterious mansion, at the end of an endless "hideous highway". The grounds are surrounded by dense and difficult-to-navigate (even with the assistance of obligatory magic words) woods. Can the player find a way into the cubic building (three floors of 3x3 rooms), treasure-hunting ensues, as well as solving a murder mystery -- if the player can avoid becoming another victim of the murderer! The clock is always running, and should the player dally, the mansion, along with its treasures, its host of resident NPCs, and the player itself, all are incinerated in a house fire sparked suspiciously (or, dare I say... mysteriously?) at the stroke of midnight.