Mega Man Network Transmission is a game in the Mega Man Battle Network series for the Nintendo Gamecube. While ostensibly based on the Battle Network series, the game is in the vein of the original series, being a platformer instead of an RPG. "Network Transmission" is also abbreviated NT, as it is in MegaMan NT Warrior, an anime series based on Battle Network.
The game takes place between the events of Battle Network and Battle Network 2, in which The WWW's leading scientist (other than Dr. Wily himself) only known as The Professor, unleashes the Zero virus on the cyberworld. Later on in the game, if a special file is found, the virus evolves to become a true NetNavi, Zero.EXE.
Lan has other qualms to deal with however, receiving an e-mail detailing Roll.EXE being unable to jack-out and trapped in the internet. MegaMan goes to save her, finding an infected Fireman.exe as the cause of trouble. Defeating him, the duo talk to Mr. Match (Fireman's operator), and learn of the vaccine being distributed to amend the Zero virus is actually the opposite, causing Fireman.exe to go berserk.
Confirming this with Lan's father, the two set out to find the crux of the problem, finding many situations of pragmatic Navi's infected and causing mayhem. Defeating all of them and returning them to their respective operators, the two eventually discover more clues leading to the remnants of the WWW. It is revealed that a powerful Navi called Starman.exe has been distributing the virus, created by Zero.EXE. Defeating Starman.EXE, MegaMan and Lan eventually engage in a climatic battle against the powerful super virus himself. However, at the conclusion of the battle, as MegaMan.EXE and ProtoMan.EXE are set to deliver the finishing blow, they discover he isn't evil, and Lan's father transforms him into a full-fledged Navi.
Their happiness is shortlived, however, when a former member of the WWW, (called only the "Professor") reveals this was all part of his scheme to revive the dreaded LifeVirus. Analyzing clues, MegaMan.exe and Lan eventually engage the second LifeVirus at the heart of the Undernet, defeating it. And with Zero.EXE's observation powers, eventually bring the Professor to justice.
There is a short dialogue at the end of the game between Shadowman and his operator, leading the plot into the video game MegaMan Battle Network 2.
Unlike previous Battle Network series, Lan is restricted from moving location to location; instead, the player uses a map screen with points of interest to travel to different levels. Levels are opened up as the player progresses through the game, with a slight emphasis on linear progression, although MegaMan can diverge "off path" to gain secret items, more battlechips and other power-ups.
Battle is done in real time, with MegaMan.EXE given the ability to jump, slide, and dodge attacks, abielt on a 2-dimensional plane. Battlechips are provided to the player through a "custom bar"; filling up as the time accumulates (the custom bar fills up much more slowly compared to the Game Boy Advance titles). When the bar is full, the player can select various battlechips, which can be provided in multiple quantities and can be used for platforming aspects as well as battle.