In Call of Cthulhu and other investigation games, the core of play is in using skills to find clues. Those lead to more clues and on, until enough information has been accumulated and the central mystery can be unraveled. A failed skill roll, though, breaks the chain and prevents the players from unearthing pivotal clues. To fix this, Robin D. Laws designed the Gumshoe System in 2006, and that powers Trail of Cthulhu (2007), which is a direct attempt to repair perceived issues with Call of Cthulhu.
Investigative skills get points, usually ranging from 1 to 4. Those points don’t represent prowess. Rather, they form a pool of personal capacity that depletes over the course of the day and is refreshed by rest. Investigative skills always succeed: you always get the core clue associated with that skill, but you can spend points to increase the degree of success. Neat!
There are some other key changes. The game moves from the fancy free 1920s to the grim 1930s. The Great Depression is a better fit for a game of nihilistic horror. The sanity system is a bit different too. Horror first depletes Stability and attacks Sanity only after the first is gone. Stability regenerates, Sanity doesn’t. Characters also have a drive, a powerful motivation at the core of their identity. This defines many of their actions. Just why is Professor Brookdale looking for forbidden artifacts and ancient ruins? Her drive compels her — literally, the GM gets to use character drives to get them to do things the player doesn’t necessarily want them to. Drives can be denied, but at the cost of Stability. Along these lines, in the opposite direction, are pillars of Stability. For every three points of sanity, your character has a person or thing that sustains them. Should those things be destroyed, they will be shaken. Should they lose sufficient sanity, they’ll cease to care. This seems to inform some of Delta Green’s ideas and generally shows how thinking about the depiction of mental illness in RPGs continues to change and evolve over time.
