Mega Man II for the Game Boy is a neat game. In it, Mega Man is back in action after Dr. Wily steals a time machine from a time-space research laboratory, and after some modifications, would go to the future and kidnap Mega Man's future self, modifying him into Quint.
So, yeah, like Dr. Wily's Revenge, Mega Man II is another mish-mash of games. The first four Robot Masters are the remaining four MM2 Robot Masters who didn't appear last game, with the other four being Magnet Man, Hard Man, Top Man, and Needle Man from Mega Man 3. In a change from last game, the last three Robot Masters get their own stages instead of being a boss gauntlet, which adds to the length. Also, Mega Man has his ability to slide, and Rush appears in all three forms (although they have to be earned by beating three of the first four bosses).
While the game makes some improvements, it is a step down from Dr. Wily's Revenge. Developed by Japan House Systems, which is a different developer than Minakuchi Engineering, the game feels a little on the easy side, and does not use the music from the NES titles, making new music that ends up being high pitched because of a rendering error. This would be Keiji Inafune's least favorite Game Boy entry, and result in the rest of the Game Bo Mega Man games being handled by Minakuchi. It is an okay game, but it's a step down from it predecessor.