It's back again with new gameplay mechanics.
Plot: Unlike the 2nd game, the intro had no cutscenes or anything explaining about the story. I had to look at Wikipedia for this.
"In the year 200X Dr. Wily claims to have reformed and works with Dr. Light on a project to build a peace-keeping robot named "Gamma." However, the Robot Masters set in charge of the "mining worlds" go berserk and make off with the eight power crystals. Mega Man is called into action, this time with a canine companion named Rush, to retrieve the crystals from the mining worlds. After the crystals are retrieved, Wily steals Gamma and retreats to his new fortress. In order to stop Wily's plan for world domination, Mega Man destroys Gamma. After their final confrontation, Wily's fortress begins to crumble. Proto Man appears in enough time to save Mega Man, but too late to save Wily, who is crushed under the rubble."
Review: This is yet another platformer intended for hardcore gamers. You get to be Mega Man and get the selection to chose 1 out of 8 stages on the stage select screen to go to. After you complete one stage, you get to pick another stage and it continues until you have beaten all eight of them. In each stage are many enemies and new, interesting obstacles you need to get over to reach the end of the stage. There is A LOT of platforming so expect to die a few times if you haven't played a Mega Man game before. At the end of each stage is a boss called Robot Masters. After you defeat a boss, you get their signature weapon. These weapons are very effective on other Robot Masters. This system never gets old for me and is a pretty fun experience.
There are new gameplay mechanics. The first is that you have a weapon besides the Mega Buster at the beginning of the game. This weapon (specifically pet) is called Rush. Rush serves you by getting you to reach hard to reach places. You can get quite a few power ups with it. The second is that you have the ability to slide. Sliding let's you go under enemy fire and go through tight spaces. It can also let you move through the stage a bit faster. Mega Man also fights a mysterious robot called Break Man (Proto Man) in the middle of some of the stages. These fights will appear in future games.
After you defeat the main eight stages, you have to revisit four of them. The stages are basically the same except you have to fight Robot Masters from the Mega Man 2 in a different form. (Nostalgia!) After you finish those stages, you get to the final run and go through Dr.Wily's Castle to eventually fight the evil scientist himself.
Closing: This is an excellent Mega Man game. It's a bit longer than the last to games so there's more to do. I'd definitely add it to your NES or VC collection.