In an upcoming episode, my crew will be placed in a position where they have to rescue their captain, an NPC. She has been kidnapped by aliens who intend to use her as a vessel for their "god", an alien whose mind was imprisoned a few thousand years previously, by another of its kind.
Part of the episode will consist of the crew persuading the imprisoning alien, who is still alive, to assist them. Assuming they can do so, it will help them. At the end of the episode, the alien will advise the crew that it wishes to avoid a recurrence, and so, the captain must die, to forestall any chance that other aliens will attempt to use her in this fashion in the future. It produces a weapon and launches this at the captain...
The trick is that someone must attempt to sacrifice themselves to save the captain's life. If they do, the disc will smash harmlessly into them -- what the alien *really* wants to know is whether someone will sacrifice themselves to save another. This is a concept unknown to him; his people exist so independently that it's significant when any two of them meet.
If they don't make the attempt, the weapon will kill their captain -- and they will subsequently learn that it can't harm anyone else. I plan to give the players fifteen seconds to write down what they do, then I'll apply those actions according to a set of reaction rolls I'll have them make earlier in the game session.
I welcome advice from other GMs or players, especially (but not exclusively) advice that answers these questions:
1) Right now, I plan some very subtle foreshadowing. Should I, or should I count on my players to do what a starfleet officer is supposed to do?
2) If you were a player, would you consider this a rotten trick? Or a fair gambit?
Thanks in advance