As the topic says, I am curious what happens if an investigator closes a portal that another investigator is trapped in? For example, player A is inside portal A and a monster surge occurs and there are now 2 brutal monsters waiting outside the gate for when they return. Player A is fairly weak and will be unable to beat these monsters in battle. Because of this, Player B decides to use his turn to go and attempt to destroy the monsters waiting outside the portal. Player B succeeds and is sucked inside the portal. Next turn, Player A uses his next movement to move out of the Other World and then attempts then succeeds in closing the portal.

What happens to player B inside the other world?

The investigator in the OW does everything as normal. At the time they would return to Arkham (to the gate that was closed), check to see if there is another gate to that world open (after all, one may have opened after the first gate was closed). If there is no way back, then the investigator is Lost In Time and Space. Note: this is the simplest case, given that this is Arkham, there will be exceptions (i.e if the investigator has a Gate box, then they can return to any open gate).

Thank you for the quick reply!