From Wikipedia
Metroid II: Return of Samus (メトロイドII RETURN OF SAMUS?) is an action-adventure video game, the second game in the Metroid series, and the only one developed for the Nintendo Game Boy. The sixth installment in the overall series story, the game was developed by Nintendo Research & Development 1 (Nintendo R&D1), and published by Nintendo in North America in November 1991, in Japan in January 1992, and in Europe in May 1992. In Metroid II, the developers added round metal shoulders on Samus's Varia Suit to differentiate it from her Power Suit, since both looked similar in the Game Boy's limited black and white color palette. The updated suit has since been a staple of the series, appearing in all subsequent games.
The story of Metroid II follows the protagonist and playable character Samus Aran, who is sent on a mission to exterminate the Metroid creatures from their home planet SR388 before the antagonistic Space Pirates obtain and use them. On the planet, Samus encounters Metroids in different stages of their evolution cycle, ranging in forms from small jellyfish-like creatures to large, hovering, reptilian beasts. Eventually, Samus reaches the Queen Metroid and kills it. Continuing through the planet's caverns, Samus finds a Metroid egg, which hatches in front of her. The creature forms a bond with Samus and follows her to her gunship, setting the plot for Super Metroid (1994).