By Mega Man 4 (1991) the formula was set, so Capcom added the one thing that would define every game after: the chargeable Mega Buster. Hold the button, release a big shot. Suddenly your default weapon mattered.

It's a bigger change than it sounds. In the first three games the Buster was a pea-shooter you fell back on when your good weapons ran dry, and the real game was managing eight ammo bars. Give the Buster real damage and the whole economy shifts: now you can clear a stage on your default gun and save the special weapons for the moments that call for them. Every Mega Man since has been balanced around that assumption, and you can trace a straight line from here to the X series.

The charge has a cost too, which is the part good players learn to hate and then love. Holding the button means you aren't firing, so a charged shot is a bet that you'll get a clean window to land it. Against a boss with a tight pattern, that bet is the whole fight.

The story got a new shape as well. For the first time the villain isn't Wily from the opening frame: the culprit appears to be the Russian scientist Dr. Cossack, and you spend four extra stages working out that his daughter Kalinka is being held hostage before the real mastermind steps out from behind the curtain. Eight Robot Masters, then Cossack's citadel, then Wily's. It's the longest game in the NES run.

This is also where Eddie shows up, the hopping supply capsule who drops a random item at fixed points, with Beat arriving in the next game. Dive Man, Skull Man and Dust Man round out a roster nobody ranks first and nobody calls bad.

Some call it the start of the series coasting. The charge shot says otherwise. It's the entry that made Mega Man feel powerful.