In a LARP, you want to be especially careful not to overplan! Decisions need to be made instantly and with as little down time as possible (ideally none). When planning the campaign, everything needs to be concise, easy to communicate and understand, and flexible (easily changed depending on players' actions).
Focus on milestones & information, not specific plot items or outcomes. (The players will create the actual story. This is the “interactive” part of interactive storytelling.)
There are many ways to design a campaign. This page describes one way to approach it, but other approaches are possible.
Situation: one sentence that describes the plot line (but not the resolution).
Example: “A group of necromancers are revolting against the king for outlawing their craft.”
Each session should lead to the next in some way with some “token”, which is a piece of the situation (part of the sentence). Players shouldn't have the full situation until the end of the next-to-last session. The last session is where the situation gets resolved.
Example:
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Milestone: Players discover zombies.
Token: all zombies are marked with a common necromancy symbol.
“A group of necromancers are revolting against the king for outlawing their craft.”
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Milestone: Players find necromancer.
Token: necromancer is part of a larger group.
“A group of necromancers are revolting against the king for outlawing their craft.”
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Milestone: Players encounter skirmish between necromancers & law enforcement.
Token: contact with either necromancer group or law enforcement.
“A group of necromancers are revolting against the king for outlawing their craft.”
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Milestone: Players talk to either necromancers or law enforcement and get the whole story.
“A group of necromancers are revolting against the king for outlawing their craft.”
Token: information about meeting/confrontation between the two sides.
This is that “next-to-last session” that sets up the final confrontation.
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Final Milestone: Players aid one of the two sides and the situation is resolved (probably in a ginormous battle) in favor of whichever side the players chose.