Call of Cthulhu features a more grounded scenario for players to ingest than many RPGs. Without nary a dragon and flaming, jewel-encrusted claymore of doom in sight, this adventure is a more realistic affair -- at least partially -- featuring various different characters who are relatively normal individuals. Normality really is the premise within this adventure, as demonstrated by the various classes available. Players can be academics, dilettantes, journalists and detectives.
So, yeah, relatively normal chaps.
But that's where the normality ends, as their purpose within this world isn't quite as ordinary as the protagonists seem -- rather, it's considerably the opposite. Here, the scenario is one of terrible darkness and suspense where insanity and fear combine to create a suspenseful, horror-filled scenario. Players are attempting to defeat a cruel evil that is beyond space itself, which is invulnerable and will drive them insane. All of this is based off of H.P. Lovecraft novels, putting the cherry-on-the-top of a spooky cake.