This time, Earth has been invaded by a new group of robots called 'Stardroids'. These robots seem impervious to most weapons, including Megaman's Mega Buster. Dr. Light has upgraded it, dubbing it the 'Mega Arm', allowing Megaman to fire a charged fist, much like his Hard Knuckle from MM3.
Not much I can say for graphics and sound, other than the fact that they're just as good as the third and fourth GameBoy games.
The two big innovations to this game are the Mega Arm, as mentioned above, and a new helper animal, a cat named Tango. The Mega Arm fired a large fist when fully charged instead of a buster shot, which travels a short distance before returning. You can acquire power-ups for it that allow you to increase its range and power. Tango, when summoned, turns into a spinning buzzsaw and bounces around the room, destroying anything in his path. Unfortunately, he's vanished after this game and hasn't been seen since. Like the last game, you can also find P-chips to bring to Dr. Light in exchange for goodies.
This game is certainly an excellent game as well, and is worth finding, if you can do so that is. Along with Megaman 4GB, 5 is perhaps the most rare of the GameBoy series.