The House of R’lyeh contains five Call of Cthulhu scenarios that follow or expand upon events in five of H.P. Lovecraft’s stories: “Pickman’s Model”, “The Haunter of the Dark”, “The Hound”, “Arthur Jermyn”, and “The Nameless City”. Set in Boston, Providence, the British Isles, continental Europe, and the Middle East, none of the scenarios need be played at set dates or in a set order, but they could be run in the order presented to form a loose campaign using optional link between scenarios to draw investigators from one to the other.
Alternatively, the scenarios may be used to supplement classic Call of Cthulhu campaigns such as The Shadows of Yog-Sothoth which suggests that its component scenarios should be interspersed with others.