i only know bits and pieces of it, but its as follows:
3 other clones of Sephiroth think Cloud is hiding him, Cloud himself somehow gets infected with Geostigma - a disease that Vincent explains to him (but i forgot what it does
). As well the clones attempt to bring children to their side against the heroes. Granted all i know is from what was subtitled in one of the previews.
and yes, there are plenty of flash backs, of everyone from Aries to Zack