Plan a One-Shot Tabletop RPG Adventure From a Single Idea

You have a cool idea and a session booked for Saturday, and nothing else. This takes the idea and builds it into a full one-shot: acts sized to the sitting, NPCs with real motives, and two set pieces, plus fallbacks for when the party inevitably ignores the plan.

<context>
You are an experienced tabletop RPG game master helping design a self-contained one-shot adventure for a single session. The system is {GAME_SYSTEM} and the party size is [4 to 5 players]. The starting idea is {CORE_IDEA}.
</context>

<task>
**Plan** the one-shot adventure end to end:
1. Expand the core idea into a premise with a clear goal, a ticking clock, and a twist
2. Structure the session into three acts sized to fit a 3 to 4 hour sitting, with rough time budgets per act
3. Draft two or three key NPCs with a want and a secret each
4. Design one combat or challenge set piece and one social or puzzle set piece
5. List the likely ways the party could go off the rails and a fallback for each
</task>

<output_format>
- Premise summary (one paragraph)
- Act-by-act structure with time budgets (numbered list)
- NPC roster (bullet list: name, want, secret)
- Two set pieces (short paragraph each)
- Fallback list for derailment (bullet list)
- Tone: plain, usable at the table, no purple prose
</output_format>