Zack: A Prelude

Zack is traveling with a half-conscious Cloud in the back of a truck. Zack talks about his current situation. They come to Midgar and Zack notices MPs approaching. You battle the platoon of MPs and Zack returns to Cloud. After being shot he flashes back to the events prior.
Zack has recently joined soldier and is accompanying Sephiroth and a bunch of MPs to Nibelheim to investigate strange occurrences at the reactor.
Zack wanders around, exploring the town and eventually, he and the others are off with Tifa toward the reactor. During the trip, one of the two accompanying MPs is lost after a fall off the bridge. How they later returned to town is anyone's guess.
While Cloud watches Tifa outside, Zack learns of the crimes of Hojo and watches Sephiroth begin his descent into madness. He later tells all of this to Cloud, venting his anxiety and frustration.
The big to-do of this quest is fighting Sephiroth. Win or lose (explainable as this is mere memory, which can be deluded), Zack pulls Cloud out of the Mako tanks and returns to reality where he is shot to death.

The White Sword: Sephiroth

Sephiroth learns of his creation and creates a fantasy of grandeur in which he is the sole heir to the planet as Jenova's "son." Deluding himself into believing Jenova was an Ancient who settled the planet, he hates all mankind for what they've done to "her" and the planet and the rest of the Ancients.
So his goal is to unlock the Ancient's Temple, obtain the Black Materia, and summon the destructive Meteor to cripple the planet.
When the planet's spiritual energy converges on the wound to heal it, Sephiroth will be their to inherit it.
However, along the way, he'll battle Avalanche and Shinra.