The Consuming Shadow follows one of the Playable Characters as they attempt to repel an invasion by a hostile Ancient One into our reality. By various occult means, the protagonist has learned of an impending "convergence" at Stonehenge, and must travel the now evil-infested United Kingdom to understand which of the Ancient Ones is attempting to invade, and to discover the Ritual of Banishment that might repel it.
There are five known Ancient Ones - Byarla, Chzo, Gnix, Kthoth, and Yushg. Each game will have three of these randomly assigned to be active and in various roles; one of the beings is the Invader, another is assisting it in its invasion of our universe, and the third is opposing this invasion, albeit not to protect humanity but instead for its own unknowable reasons.
Through various clues and notes, the player must discern the characteristics and motivations of the invading entities and learn which four runes must be incanted to perform the Banishment Ritual. These goals must be accomplished in time to reach Stonehenge and seal the portal before the Invader arrives - and preferably without the protagonist losing their sanity entirely and committing suicide.