The story of Mega Man & Bass varies extremely depending on which player character is chosen. It begins one year after the events of Mega Man 8 when a robot villain named King breaks into Dr. Wily’s laboratory and then the Robot Museum to collect the data blueprints for the creations of Dr. Light. Dr. Light alerts the hero Mega Man that he must go at once to the Robot Museum to confront this new enemy. Meanwhile, Bass (Mega Man’s rival and Wily’s greatest creation) hears of the new criminal’s appearance and decides to prove himself the stronger robot by defeating King.
Proto Man is the first to arrive at the scene. King divulges his plan to him; he desires to create a utopia in which robots rule the world over humans. To accomplish this, King seeks to create an unstoppable army using the data and invites Proto Man to join him. Proto Man refuses and attempts to attack, but King counters and slices his body in half. Proto Man then teleports back to the lab for repairs while King escapes with the data, instructing his minions to handle the heroes. With their own motivations, Mega Man and Bass set out to put a stop to King’s plans.
The gameplay in Mega Man & Bass is similar to earlier games in the series. The player is tasked with completing a series of action-platform stages while overcoming obstacles, solving minor puzzles, and battling enemies. Destroying the “Robot Master” boss at the end of a stage lets the player acquire its special weapon. In previous games, the player generally took on the role of the hero Mega Man. In this game, the player can choose to start the game as either Mega Man or Bass. However, whichever character is picked must be used for the rest of the game and cannot be changed.
Mega Man is able to charge his shots to make them more powerful and has the ability to slide along the ground. Bass is able to rapidly fire his arm cannon in eight directions, though shots cannot be fired while moving or pass through walls unless a certain upgrade is obtained. Bass is also able to double-jump (jump a second time in mid-air) and dash along the ground (similar to X in Mega Man X). Performing both simultaneously lets him cross great distances by doing a dash-jump.