Canis Mysterium is an adventure scenario for the Call of Cthulhu roleplaying system designed to be played in one or more gaming sessions. This scenario is suitable as an introduction to a new campaign or can easily be worked into an established storyline. As written, this adventure is set in and near Arkham of October 1930. In "The Werewolf of Coldwater Falls", the investigators are called upon to research the strange psychology behind a man exhibiting canine behaviors, discovered with the shinbone of a missing girl dangling from his maw.

Several scenario seeds are offered to keepers who wish to expand and continue the events of this gaming adventure, perhaps giving birth to a brand new campaign or providing handy subplot sessions to pre-existing campaigns.