As with all the other Mega Man games, the blue bomber commits robot-on-robot violence to get medicine for a robo-flu. That scientist from early adventures may or may not be relevant. Proto Man guest stars.
Same Mega Man goodness: jumping, shooting, climbing ladders, pit trap goodness. Mega Man doesn't have slide or charged shots. Proto Man can deflect bullets, slide, and shoot charged shots but can only shoot 2 pellets. Bass (DLC) can shoot in all directions and dash, but he stands still when shooting.
For this (questionable) longplay thingy, I go through the game with all three characters. Mega Man is cleared on Normal, Proto Man on Easy, and Bass on Hard. Boss weakness is abused (lame, I know.) and the game owns me on Hard Mode.
There's no Endless Stage DLC and Special Stage DLC. Sowwy. =(