Meh. Maybe we won't.

[Quasqueton]

The immensely powerful being tells the PCs, "I need the mcguffin from the dungeon of doom to complete my ultimate plan. You will go get it."

The PCs reply, "What if we don't want to?"

What does the immensely powerful being do?

Quasqueton

 

[Crothian]

Why is the immensely powerful being asking the PCs to do it and not doing it himnself? Why would he outsource such an important mission?

It really depends oin why he is asking them in the first place really.

 

[Jdvn1]

"All right, I'll give the really cool items/extravagant riches to someone else."
"All right, I'll give someone else a really cool fortress."
"All right, I'll eat/destroy you."
"All right, I'll eat/destroy your family."
"All right, I'll imprison you until you change your mind."
"Infidels! Tell me why I should not smite you now."

 

[der_kluge]

Yea, I like the "I've told you too much, now you die!" approach.

 

[Voadam]

The immensely powerful being tells the PCs, "I need the mcguffin from the dungeon of doom to complete my ultimate plan. You will go get it."

The PCs reply, "What if we don't want to?"

What does the immensely powerful being do?

Quasqueton

"Then I will curse you so that all the beer you can ever get is green."

Or depending on their temperament

"If you do this for me, all the beer you ever recieve will be blessed and turn green."

The immensely powerful being is a 2 hp leprechaun with polymorph any object at will, right?

 

[Timeboxer]

Well, if you really really really need them to do it.

A) Geas all of them. This may take awhile.
2) Plane shift to another plane, gate in the party, and force them into (fairly paid contractual) service.

 

[Cergorach]

Group of naive PCs: "What if we don't want to?"

The immensely powerful being: "You'll learn to live in interesting times." and grins evily.

 

[Ashy]

Make the alternatives horrible. Have the supreme being show them a vision of what the world will become if they do not undertake the quest. Apparently, only they are able of doing it, so only they are capable of preventing the calamity shown to them.

Also, when showing these harrowing scenes, it's always a good idea to show the characters views of what will happen to their own loved ones and not just the faceless masses... Heck - show the characters their own futures!

 

[Gez]

"Well, if you don't want to, I wash my hands of all this. I am able to shadow-shift out of this doomed cosmology, not you, so it's not me who cares the most about your silly appetizer-world."

 

[TheAuldGrump]

It depends - if it is the players who are going 'oh my god! What cereal box did you get this one off of the back of?' then the thing to do is find out what they actually want to play. For some reason a lot of players resent a railroad ride and being forced to do things. I have been on both sides of the DM screen on that one, and I can't say as I blame them.

The Auld Grump

*EDIT* Mind you a powerful being bribing the PCs is a completely different story.