Metroid MSU-1

In the year 2000 C.C. (Cosmic Calendar) the leaders of various planets united in order to form a congress that became known as the Galactic Federation in an attempt to construct a fair and structured universe that would enable society to grow and prosper. Under the guidance of this new federation, the individuals of the planets began to associate with each other, and a new civilization began to develop. Various leaps in technology for transportation were created, such as interstellar spaceships, and society flourished through the use of such expansion.

Note: According to Metroid and Metroid II: Return of Samus's manuals, the year of the Galactic Federation formation was the year 2000.[7][8] In Metroid: Zero Mission's manual, the year of the Galactic Federation formation was 2003.[9]

At this time, Earth came into contact with individuals from these other worlds and the advanced technology that these people possessed was shared with the humans of Earth. All seemed well in this new society. However, devious groups known as "Space Pirates" began to attack the spaceships in the hopes of looting valuable goods from them and striking fear in the hearts of the people. In order to counter these attacks, the Federal Bureau set up a new combat force known as the Federal Police. Yet the pirates were extremely difficult to battle in the depths of space, even with the advanced technology of the Federation. Thus, they recruited a group of courageous individuals who became known as "Space Hunters", equipped with the best weapons available. The Federation provided huge financial bonuses to the Hunters in reward for hunting down and destroying the pirates.

t is now the year 20X5 C.C., and the universe has continued to develop while battles still rage throughout space. Recently, reports have indicated that a spaceship is traveling with a capsule containing an unknown life-force from the deserted planet SR388. This ship has been attacked and seized by the Space Pirates. While research about this life-form, currently in hibernation, is incomplete, it is known that exposure to beta rays for 24 hours will cause it to multiply. Some scientists believe that this life-form may have been the cause of destruction of life on SR388. The Chozo called this being a "Metroid" and the mere thought of it being in the hands of pirates is utterly disturbing to the human scientists. If the Pirates learned how to multiply this organism and use it as a biological weapon, the cost of lives could be overwhelming. The Federation launched search teams to find the Pirates and were fortunate to discover that their base was located deep within the planet Zebes. However, none of the forces are strong enough to take the pirates down.

During this outside battle, operations to multiply the Metroid within the Space Pirates' headquarters were soon becoming a reality. Desperate for a solution, the Federation decided that the only option left was to attempt to infiltrate Zebes and destroy the leader of the Space Pirates, "Mother Brain." To make matters difficult, the structure of the planet Zebes is a natural fortress that consists of a large maze. Scattered throughout the maze are various traps and allies of the Space Pirates. This mission clearly requires a special individual to complete, and so the federation has selected the most dominating Space Hunter of the entire organization: Samus Aran.

Samus, though human, has a cybernetic suit built by the Chozo, thus providing her with a strange power. She has, to this date, completed many missions considered impossible, and her suit has the power to withstand lethal attacks, while providing many itself. Samus makes her way through the fortress-planet Zebes, collecting weapon and health upgrades, advancing to new areas, defeating Kraid, Ridley, and Mother Brain, and, in the end, destroys all Metroids on Zebes.