The same story of Episode III, changed in tiny ways so you get some really wacky possibilities.
Consider:
If Mace Windu was sent to rescue the Chancellor?
If Anakin had led the campaign to kill Grievious?
If Yoda had bested Sidious?
and the big one...
WHAT IF ANAKIN HAD MISSED MACE WINDU AND KILLED PALPATINE?
These points should make for some intresting discussion