"D-59" is the official Cronos designation for the incident in which the organisation lost the recently-discovered Guyver Units. The units were stolen by a Zoanoid test subject from Cronos Japan who intended to exchange the alien devices to a mysterious client for having his Zoanoid optimization reversed, not knowing that the process was permanent.
The day after the theft, Cronos sent its agents into the Narisawa area to find the escaped test subject and take back the stolen Guyvers. They found their target and the team's captain, a Gregole, transformed and attacked. Unable to match Gregole's strength, the "Marmot" used his last resort: he revealed a grenade and used it to blow himself up and the Guyver units. The test subject was killed but the Guyvers were scattered across the mountain range. The Cronos agents recovered one of the units, though it had recieved damage from the explosion. The other two survived and one of them would be discovered by high-school students Sho Fukamachi and Tetsuro Segawa.
Sho would accidentally trigger the Guyver unit and it bonded with him. As the symbiote's tendrils enveloped his body, the terrified Sho staggered and fell into Narisawa Lake. Moments later, Tetsuro would be confronted by the Cronos troops. Not knowing who they were, Tetsuro was shocked to find a hideous monster amongst the soldiers. Since Gregole had been seen, he moved in to kill the witness, but another inhuman figure suddenly emerged from the lake: the Guyver.
Sho had briefly lost consciousness after bonding with the Guyver, so he was not in control when he fought against Gregole. Using only its bare hands, the Guyver snapped Gregole's arms and neck, killing him. The other Cronos soldiers retreated and Sho regained consciousness seconds later, seeing his reflection in the lake. Confused and frightened, he cried out in defiance. It was then that the bio-boosted armour suddenly disengaged, appearing as a ghostly figure in front of Sho and Tetsuro that sank into the lake and disappeared. From that point onward, Sho's life was forever changed and he would find himself regularly being confronted by Cronos' ferocious Zoanoids and forced to fight for the lives of his friends and later for the entire world.