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Zack fought for Shinra for several years trying to gain his right to be a hero like Sephiroth. He fought along side his new friend Angeal, the original owner of the Buster Sword, and ended up having to fight him (there after gaining the Buster Sword.). He later met cloud, who was part of the Shinra security team and not Soldier, and became close friends with him. After a while of chasing Angeal and Genesis (both were friends with sephiroth), he had to fight Genesis and ended up pretty much killing him.

With Angeal and Genesis gone the only one of the three friends left was Sephiroth. After visiting the Nibelheim reactor Sephiroth goes crazy for the truth, and finds that he was more or less an experiment created by his own father, Hojo, and mother, Lucrecia. He also finds out, as Zack did earlier, that Angeal and Genesis were from the same experiment that came about when Hojo decided to use the power of the Jenova cells to inhance human capabilities and to create the perfect soldier. He starts to become entirely insane and burns all of nibelheim to the ground and kills almost all of the people who live there.

He then left for the reactor, Zack, Cloud, and Tifa followed. Tifa came in first and was hurt pretty bad, Zack came in and fought against sephiroth, winning at first, but he eventually gets knocked back and flung down onto the stairs outside of Jenova's tank room. Cloud rushes in and Zack tell him to kill sephiroth, so Cloud takes the Buster Sword and rushes forward, stabbing it into Sephiroth and busting Jenova's tank open. Sephiroth was still alive after this and began to walk from the reactor with Jenova's head, but Cloud wouldn't let him leave. Sephiroth stabbed his sword through Cloud, and Cloud uses it to throw him off the side. Sephiroth was there after believed to be dead...

Cloud passed out on the stairs next to Zack, and Hojo found them lying there and had the Shinra troops put them in a Jenova cell bath for his experiments. Zack then awoke some time later and busted free of the tank, taking Cloud from his and returning to Nibelheim from Shinra mansion. There he found the Charred remains of Nibelheim rebuilt into an exact replica of Nibelheim as part of some cover up to keep Sephiroth's insanity a secret. Zack escaped, but barely and managed to make it to the dessert like land just outside of Midgar, where he fought against Shinra security troops for his life. Three Shinra troops that were left shoot him and he collapses to the ground.

Cloud crawled over to Zack a short while after the fight and Zack gives him the Buster Sword, and a memory, which the Jenova cells distorted and tried to make bond with what all he remembered to make it seem like Zack did not exist, but in fact he was the Soldier 1st class that he had hoped to reach so that he could prove to everyone in Nibelheim he was a hero like Sephiroth. Zack makes Cloud live enough life for both of them and Cloud watches Zack die. He later, after Final Fantasy VII, uses the Buster Sword as Zack's grave stone and a marker for his memories.