A man named Trigger once lived on the planet Elysium. There, Trigger served under the Master System as a Purifier Unit: a hunter of irregular, or aberrant, units that threatened the existence of the Master System. One day the Master System ordered Trigger to go with him down to Terra, so that the Master System could observe the Carbons, artificially-created humanoids that the Master System had watched over for 3,000 years, first hand, despite the fact that the Master System could not survive long outside of Elysium.
Down on Terra the Master System became corrupted and came to believe that although the Elysians had lived comfortably on Elysium for so long, that Elysium was cold and lifeless, and that despite all of the pain and hardships the Carbons suffered and endured through, they were happier. Believing that it had made a mistake with Elysium, the Master System gives its genetic code to Trigger for protection and orders him to delete all of the data in Elysium's Library.
Trigger attempted to follow through with the Master System's final order but his actions understandably got him labelled as an aberrant unit, making him an enemy of Elysium. Yuna, the Mother Unit of Terra, opted to neither help or fight against Trigger, while Sera, the Mother Unit of Elysium, travelled to Terra herself in an attempt to stop Trigger and obtain the genetic code of the Master System. Sera and Trigger had an all out fight that resulted in a stalemate. Trigger's original body was nearly destroyed in the battle, which caused his parameters needing to be reset to allow him to heal, which resulted in Trigger transforming into a baby with no memory of the past. Before this happened, Trigger transferred what essential data he could, including the Master System's genetic code, into something called Data in an attempt to protect it all from Sera. Yuna took this opportunity of Trigger and Sera's exhausted state to place the baby, Data, and Sera into stasis fields. The baby and Data were sealed inside ruins on Nino Island, and Sera and her servant, Geetz, were sealed inside Forbidden Island.
Many, many years passed before Barrell Caskett explored the ruins on Nino Island, without permission, and discovered Data and the baby in a field that disappeared when he touched it. Barrell, unable to reveal that he had found them due to illegally searching the ruins, opted to take them under his care. Roll, Barrell's granddaughter, insisted that the baby be named "Mega Man" after one of her favorite videogame characters. In the Japanese version of the game, he is named by Barrell, not Roll, and is named Rock as a pun with his granddaughter's name. It is unclear where the surname "Volnutt" came from. Mega Man was then raised as Barrell's adopted grandson, who took on the life of a Digger once he came of age.