If you were to do a Coles Notes version of the Metroid storyline (and not have it turn out like this), I think there'd be four games you would absolutely have to include in order to get a relatively full picture of the storyline: Metroid, Metroid II, Super Metroid, and Metroid Fusion. What's weird about that (and in stark contrast to, say, how Sony treats the PSP) is that two of the main games in the series are handheld. I had never really even considered Metroid II as being as important as it is, mostly because I was under the impression that the Game Boy wasn't really the platform for serious games. I was wrong wrong wrong.
Metroid II is an excellent game that is propulsive and confusing, highly detailed but in black and white. Ignore the "Return of Samus" subtitle – this is a game all about metroids, those brain-sucking monsters who permeate the entire series, but nowhere more than here. Although the Prime series is fantastic and contains some of the best games ever made, it's hard to get around the fact that the series is more or less self-contained. The saga of Phazon begins and ends with the Prime games, whereas the Space Pirates and the metroids are everywhere in the series, and nowhere (save maybe this game's sequel, Super Metroid) is this more fleshed out through gameplay than in Metroid II.
Having defeated Mother Brain and destroyed the planet Zebes (and gone on your Metroid Prime excursion), you now are in charge of heading to SR388, the home planet of the metroids, in an effort to eradicate them all. Gameplay, then, is very similar to the original Metroid, except that there's always something pushing you forward, and that's killing all the metroids. There's even a countdown on the right hand side of the screen to remind you how many you have left, and it was this aspect of the game that I found really compelling. It kept me motivated and gave the game a sense of urgency not really seen in the rest of the series.
Metroid II is really a culmination of all of the events of the original Metroid, as well as the launching pad for the rest of the series (narratively, anyways). In that way, it makes the game feel really, really important, and it's strange to me that I'd never played it before now (not having a Game Boy probably would have been the reason). It's even more impressive because the game, while offering significant changes to the gameplay, also encapsulates the Metroid experience so well. SR388 is easily the most alien-feeling planet I've encountered in the series, and the black and white palette of the Game Boy highlights the dark atmosphere perfectly. In fact, this game made me wish that more games were produced in black and white intentionally (that aren't Madworld. Dear god, not Madworld.) because it can be used to great effect.
Don't get me wrong, I love the music from the Metroid Prime games, but Metroid II has my favourite soundtrack from the entire series. I had no idea that the Game Boy's tone chip could create such beautiful beeps and boops, but Metroid II proves that video game music is not only defined by technical limitations, but can even thrive under these conditions. What really impresses me is the way in which the sound effects interact with the music, specifically when Samus' health goes down. Instead of the usual, annoying "beep beep beep" sound that plays in most of the other games in the series, there's a small musical piece that plays in and around the music designed for each area of the planet. It really works well and makes the game a small musical miracle.
Small spoiler alert (I mean, it's not much of a spoiler if you've played Super Metroid): Samus doesn't kill all of the Metroids. There's one left at the end of the game, a baby, who Samus doesn't have the heart to destroy. Instead, she gives it to a team of Galactic Federation scientists to study, leading to the events of Super Metroid. It's this one act that I think defines the series and gives it a human texture. Plotwise, it's also easily the most important moment in the series. Speedrunners might be trying to blast through the game so that they can see Samus in her undies, but this is the real end of the game, and it's powerful to say the least. To put that kind of experience on the Game Boy was a major risk, but one that I'm glad Nintendo took. It's time to end the delineation that handheld games can't be as full and rounded as a console game, because even though it's almost 20 years old, Metroid II proves that that's simply not the case.