His story, in short
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Leaving Gongaga
He heads for Midgar with no plan beyond "join SOLDIER, become a hero," and somehow makes it work.
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A puppy in SOLDIER
As a 2nd Class he trains under Angeal Hewley, who keeps telling him that dreams and honor count for more than rank. Angeal calls him a puppy. It isn't entirely unfair.
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Wutai, and the deserters
During the war with Wutai, Genesis Rhapsodos walks out of SOLDIER and takes a lot of people with him. Then Angeal disappears too. Zack is sent to find out why, and learns more about how SOLDIER is made than he ever wanted to.
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The church in Sector 5
He falls through a church roof in the slums and lands in a flowerbed next to Aerith. They start seeing each other. He helps her build a wagon so she can sell flowers all over Midgar.
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First Class
He makes 1st Class, and in the end has to face Angeal himself. What Angeal leaves him is the huge sword he never liked to swing.
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Nibelheim
A routine reactor job with Sephiroth and a quiet infantryman named Cloud Strife goes as badly as anything can. Sephiroth learns where he comes from, loses his mind and burns the town. Zack goes after him and loses. Cloud is the one who ends it.
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Four years in a basement
Shinra buries the incident. Professor Hojo keeps Zack and Cloud in the basement of the Shinra Mansion as test subjects for about four years.
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The road to Midgar
Zack breaks out and carries a half-conscious Cloud across the wasteland toward Midgar and toward Aerith. On a cliff within sight of the city, Shinra's army catches up with them. He fights alone, and before he dies he gives Cloud the sword and asks him to live for the both of them.
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What comes after
Cloud walks into Midgar carrying pieces of Zack's life as if they were his own. That's where Final Fantasy VII starts.