When born in a place like Gongaga, the rest of your life was bound to be confined to its limits. Big dreams and ambitions could only be attained by those that dared to cross the borders, the ones who refused to be restrained to a country living—Safe and steady but lacking the fulfillment that the heart of a young Zack Fair craved. At the age of thirteen, he fled his home and set out to Midgar, where he joined the elite military forces of the Shinra Electric Power Company known as SOLDIER. Taken under the tutelage of Angeal Hewley, the young man reached the rank of SOLDIER Second Class when he was sixteen, where he excelled and was even recommended by his mentor for the position of First Class. To prove his skill, he was deployed to Fort Tamblin in Wutai during the war of the eastern island-country against Shinra's dominion on its land.

Determined to see his dream of becoming a hero as a reality, Zack demonstrated his prowess. But when being attacked by unknown enemies revealed to be Genesis copies, his mentor and friend, Angeal, disappears. Despite of the famous war-hero Sephiroth's certainty on how the reason for this was a desertion, the Second Class SOLDIER member refused to believe it so. With this event, Zack is introduced to glimpses of the darkness within Shinra, learning of its obscure secrets through various confrontations and the loss of his friend and mentor. All of it robs him of the joy he once thought would be had when finally attaining the long desired promotion to SOLDIER First Class. Joining forces with the Turks and Sephiroth, they learn of Dr. Hollander's involvement along with information regarding Project G. But when Angeal attacks Zack and effectively escapes, the younger man falls and crashes through the ceiling of Sector 5 Slum's church; where he's woken by Aerith Gainsborough.

After offering to pay her back with one date, he suggests she makes a profit out of the impressive flowers she grows since they are a luxury in the barren Midgar. Following helping a boy recovering his wallet in the Slums, and gifting Aerith a ribbon, their date is interrupted when Zack is called back to Shinra. Time afterwards, sent on assignment to Modeoheim, he meets who would become his best friend, an infantryman named Cloud Strife—They bond once realizing they share origins as 'country boys'. When reaching the town, he confronts Genesis and subsequently assumes him dead, hurrying after Hollander to find him and Angeal. Battling his former mentor, Zack defeats him and before dying, he is handed the Buster Sword that serves as a symbol of honor. Profoundly affected by the grief of Angeal's death, the man he deeply admired, he finds comfort in Aerith. Now one of the last First Class SOLDIER members, he becomes the forces' emotional heart, inspiring other members.

Amongst his assignments, Zack joins fellow SOLDIER members Essai and Sebastian along with a Turk to stop an AVALANCHE in Icicle Inn. But the two are captured and although he commands a rescue mission for the pair, they were already too far gone in the experiments performed by AVALANCHE's Fuhito and die. Sent to Costa del Sol for a forced vacation under the watch of the Turk Cissnei, he is attacked by Genesis copies which proves to be the beginning of another conflict. When copies break Hollander out of Junon, he is sent there to control the threat but the scientist escapes regardless, and he's given permission to return to Midgar to see Aerith. Monsters swarm the city, having been set free. Before undergoing the next mission, he and Aerith build a wagon to sell flowers together but he parts afterwards for Nibelheim, assigned to investigate a malfunctioning reactor. Sephiroth and Cloud are a part of the mission but what was thought to be another smooth investigation ultimately spiraled into disaster when Jenova is discovered and Sephiroth learns of his own origins.

The once revered hero burns down the town but is confronted by a young Tifa Lockhart, who's badly wounded when Zack finds her, proceeding to try and stop Sephiroth and failing—It's Cloud who manages to throw the now manic man down the reactor's core but having suffered serious injuries, they all fall unconscious and are taken as test subjects by Hojo. When Zack finally recovers full consciousness, he and his best friend are inside tubes of mako, with the latter having fallen victim to toxicosis. Dragging an unconscious Cloud along, he breaks free to find the town was rebuilt and Shinra troops are now hunting them down. For a year, the two flee from the company with the help of Cissnei and Director Lazard, while confronting and defeating Hollander and Genesis. On their way to Midgar in the back of a truck, Zack shares his plans of becoming mercenaries together to Cloud. Before they can finally reach the city, however, the Shinra army awaits in ambush and though he stands his ground for a long time in battle; the man collapses and imparts his last words along with his sword, to Cloud.

At least, in one world, so it would be...

In another, the last bullets would not hit their target and Zack would walk away, alive. Together with his best friend, they would reach Midgar, being received by the news report of part of the city's destruction by a 'tornado'. To his surprise, the image of Aerith is amongst the survivors recovered, which prompts him to act and try to save her from the clutches of Shinra. Thanks to Red XIII's help in bringing down the chopper, he successfully fought off the troopers and left with an unconscious Aerith. In the slums, he finds a shack to hide and tend to both as best he can in the complicated circumstances they're thrown into. Sending a note to Elmyra Gainsborough, Aerith's adoptive mother, the older woman comes by to bring her daughter back home now that Shinra has stopped their surveillance on it, letting Zack bring Cloud along as well. 

Living there for the time being, the ex-SOLDIER bonds with Marlene while helping Elmyra take care of her unconscious daughter and his mako induced, comatose friend—All the while trying to make sense of the world around him. Meeting with Biggs, they discuss the unusual and unlikely circumstances of their escape from death, quickly developing a friendship of sorts. Zack thinks the one responsible for their survival is Aerith with her Cetra gifts, but thanks to Marlene, he learns that Sephiroth is after the flower peddler and that only Cloud can save her if he wakes up in time. Determined to save them all, he ventures into Shinra HQ to try and seek answers, where he is met with a decision on whether to stop Biggs from blowing up another reactor or find Hojo. 

In different worlds, his decision will change but he will ultimately be met with his fate: to be confronted by a platoon and die. However, while pondering on his decision by the entrance of the Sector 5 slums Church, Sephiroth makes an appearance. Though Zack is ready to confront him, the once hero opens a rift with the Whispers and sends the ex-SOLDIER through it. Darkness envelops him but gives way to bright light, seemingly bringing with it yellow petals of Aerith's favorite flower, beckoning him forth. He wanders in the midst of seemingly endless white, until the frozen figure of Cloud appears. When he touches his best friend, Zack is transported to fight alongside Cloud against Sephiroth for a short time. Soon, the one-winged angel sends him away to another world that is about to meet its end, where he battles a manifestation of Sephiroth simultaneously with Cloud in another world. 

When the fight is won, he knows his time is up and falls down into a pit of the Whispers. In another world, he wakes up inside the Sector 5 slums Church, fully aware that all that transpired was not a dream. 

With renewed resolve, Zack is determined to find a way to have their worlds merge once more.