I know that the number of investigators in a game affects things, like the monster limit and the successes for the ancient one battle. But what happens when investigators come and go?

For instance; do retiring investigators stack with their replacements for determining the number of investigators?

what happens when in a three player game a fourth player turns up and brings in a new investigator, or the opposite happens; a player leaves early to go home and his investigator is taken out of the game?

The rules state that the game can allow the introduction of new players and investigators, or the departure of said players and investigators at any time. The rules don't really state, (at least i cant find anything), on how this impacts the games mechanics derrived from the number of players / investigators.

The closest thing in the Rules that I know of is located in the Final Battle section about how you have to have successes equal to the original number of Investigators, in regards to Devoured Investigators.
So you could say that everything stays in relation to the number of players that started the game.

In the case of someone leaving early from the game, have another person take over that Investigator so you don't have that problem. Heck, you can "pass" around control of that Investigator between turns if it is too much of a burden on any one player or as a group decide what that Investigator does.

If you want something to use, try this.
I would apply the rules for that new number of players with 3 modifications.
1. If the change makes the Monster Limit lower then the number of Monsters currently in Arkham, the Monsters currently in Arkham stay in Arkham, and any new Monsters go to the Outskirts.
2. If the change makes the Gate Open Limit lower then the number of Gates currently open in Arkham, the Final Battle doesn't start unless another Gate opens. Of course if the number of open Gates goes below the Gate Open limit, play as normal.
3. If the change makes the Outskirt Monster Limit lower then the number of Monsters currently in the Outskirts, the Monsters currently in the Outskirts stay in the Outskirts until another Monster is placed in the Outskirts.

The number of investigators is equal to who is "live" at the time. The number of retired investigators does nothing for the current game state. All they do is count against you ifg you wint and want to tally your score.

If people join or leave the game, that number is what counts for when new cards come up that are affected by the number of players, whether you increase the number of spawned monsters, etc.

For the final battle, the number counts as to how many investigators should go in. If an investigators is devoured as a result of a GOO's start of battle abiity, that investigator would still count when determining the total number of successes needed.

So I guess technically everyone but one could drop out of the game right before the final battle is upon them so only 1 success is needed per hit.

Seriously, if everyone were to leave right before the final battle started, I would just control their characters through the battle.

"Well chaps, Cthulhu seems on the horizon. I'm going to jet off an leave you to it then. You'll do better without my help."

Heh... Same. I body ****** quitters :')

Tibs said:

Any time we've had someone with a time crunch as it comes close to the final battle, we'll end up just triggering the final battle, so that they can die valiantly in the maw of who ever is choosing to devour us that night.

AGH! BODY SNATCHER!

The Thing In The Attic said:

Never had anyone quit just before the final battle (or the whole group but one). That would be a cheap way to game the game.