The layout of the game is similar to other 2D Metroid installments. However, Metroid II is more straightforward in the sense that advancement through the levels is not strictly controlled by item acquisition as in the other games, but by killing a fixed number of Metroids in the area. Once they are destroyed, an earthquake occurs and SR388's lava levels decrease, allowing Samus to travel deeper through its maze-like tunnels. Like the game's predecessor, Metroid II contains no in-game map. An aspect unique to the game is the Metroid detector, which displays the number of Metroids left on the planet. Metroid II is also the first in the series to have save modules, located in various points on the planet. In Metroid, saving was accomplished by dying: in the Japanese Famicom Disk System version, the player is given the option to continue or save, and in the international NES version, the player is given a password. This was the only Metroid game to not have a timed escape sequence at the end of the game until the release of Metroid Prime, which had one shortly after the game begins.
After the credits are displayed at the end of the game, the total time the player took to complete the game is be displayed. During the credits, Samus is shown running on the screen. Depending on the completion time, after the credits Samus will either continue running, jump and then pose in her suit, or jump and then pose without her suit.
Recurring items from Metroid
Metroid II takes place an unspecified period of time after Metroid Prime 3: Corruption; bounty hunter Samus Aran has defeated the Space Pirates on several different occasions at this point. Her original victory on Zebes was against their plans to use the newly-discovered lifeform known as the Metroid for evil purposes. To ensure this will never happen again, the Galactic Federation sends teams to planet SR388 to destroy the Metroids. When none of the teams survive, they contract Samus to handle the task.
During the course of gameplay, Samus destroys every Metroid on the planet. As she does so, she notes the steady mutation that each goes through, which cause them to grow from small jellyfish-like creatures into large, hovering, lizard-like beasts.
At the game's climax, Samus fights a final battle against the Metroid Queen. After killing it, Samus goes back to her ship. Along the way, she finds a Metroid egg which hatches in front of her. A small Metroid hatchling floats out of the broken shell and imprints onto Samus as its mother. Unable to commit to her genocidal mission, Samus spares its life. The two exit the tunnels, and the Metroid hatchling clears the way, helping Samus escape. Samus and the Metroid board the gunship and the game ends.
There was also a color version titled Metroid II: Return of Samus DX announced in 1999 shortly after the release of the Game Boy Color. Dan Owsen of Nintendo claimed its color-palette needs were even taken into account in the design of the Game Boy Color hardware. Some pictures were shown but the game never saw daylight despite the release of the colorized version of The Legend of Zelda: Link's Awakening.
Since Nintendo abandoned its efforts to "colorize" the game, there have been several attempts by amateurs to "remake" Metroid II. These efforts are varied, ranging from simple sprite hacks to full, complete remakes of the game. One such attempt -a self-titled unofficial Metroid II DX- adds a full Game Boy Color palette color to the game. However, the colorization method used by Metroid II DX causes some distortion to occur during gameplay, noticeably when graphics "fadeout" and also during the final boss battle. The reason for this is that the aforementioned method inserts the GBC color palette into the empty space at the end of the original game data, and adds the colorization itself during each "V-blank cycle". Special effects that utilize this cycle, such as the Queen Metroid and the "fadeout", are replaced by random tile distortions. No other noticeable effects are present with the patch. Another attempt at revitalizing the game, "AM2R", takes an entirely different method, with the author completely rebuilding the entire game engine for the PC from scratch and utilizing sprites from Metroid Fusion and Metroid Zero Mission.
Like all original Game Boy games, the original monochrome Metroid II cartridge is also compatible with the Super Game Boy accessory for the Super NES (although it is not an enhanced title), allowing the player to manually remap the color scheme to a maximum of four different colors. Additionally, the Game Boy Color (and subsequent compatible systems) automatically colorizes the game, but not true full color, as it is still limited to the same colorization techniques of the GBC and GBA.
The lack of color in Metroid II had some long-term effects on Samus's appearance. According to the GameTrailers five-part Metroid Retrospective, the Varia Suit gained the extra armor plating and bulky, round shoulderpads that have become its most recognizable traits. The change was necessary to differentiate it from Samus's normal Power Suit. In addition, rather than Samus's arm cannon and visor changing color to distinguish between "Beam Mode" and "Missile Mode", the end of the arm cannon tip moved outward and back to show that the "missile hatch" was open. These graphical changes have been used in all Metroid games since Metroid II, rather than the color changes from Metroid (however, later games used both a graphical and a color change for the Varia suit).