Scenario games can literally be based on countless different objectives but some popular themes are reenactments of World War II battles, Vietnam battles or classic films such as Aliens or games like Halo. A typical scenario event could start on a Friday evening with team assignments, explanation of objectives (recovering items on the field, capturing and defending locations, freeing prisoners, etc.) and some game play that lasts until several hours after dark with some night missions. The following day will continue the battle and last until the afternoon where the winning team is announced and other awards are given out. To keep players involved, eliminated players are typically reinserted at regular intervals so that players can literally stay involved all day.
Some scenario games require teams of players to come, but most allow for single players and small groups who are then assigned to larger teams. Most events have trained referees, chrono all guns (to make sure that nobody is shooting hot) and go over objectives and all safety rules before the game begins. When you go to a scenario game, make sure that you come prepared with all the equipment you'll need for the day.