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Metroid: Zebes Invasion Order (メトロイド ゼーベス侵入指令 Metroid: Zebes Shin'nyū Shirei ) is a Metroid gamebook that was published only in Japan in December 1986 by Futabasha. Written by Nobuyuki Shioda and illustrated by Sumi Arisaka, it is the fifth book in the Famicon Bouken Game Book series. The book contains the earliest depiction of Samus Aran's face and Gunship; the former is a secret in the original game, while the latter is not present. The book loosely follows the plot of Metroid, released earlier that year. It has never been officially translated into English, though an unofficial translation is currently in progress by MetroidLuver13. Some elements from the book, most notably the post-Mother Brain storyline, were later reused for the remake Metroid: Zero Mission.
The Space Pirates, so-named after the pirates on planet Earth, have been working in secret, and the Federation Police (a sect of the Galactic Federation) want to stop them. In this book, the Space Pirates are depicted as humans, rather than the alien creatures they were depicted as in later games (and to a certain extent implied in other extension media such as the Japanese manual for the game).
A bounty was placed on the Pirates, creating a new occupation in the galaxy: Space Hunters. However, the more actively the Space Hunters worked, the more the weaker Space Pirates joined together with their stronger comrades, forming a massive organization with immense power.
As this is going on, a Special Investigations Group creates a team to conduct research on the recently discovered Metroids from planet SR388. They are known for having destroyed the entire civilization on that planet. Some of the research involved studying various stages of Metroid development via "ecology capsules", but the brunt of the study involved Beta-Ray exposure to "inactive", seemingly dead Metroids. It was found that in a trial run on one particular Metroid, developed from a capsule, within 24 hours it would come back to life after such exposure. That Metroid, unfortunately, annihilates the entire research team, leaving the facility in ruin. The Federation goes so far as to send in the entirety of the 3rd Division of the Federation Space Army, who inevitably eliminate this Metroid.
Later, the Space Pirates manage to steal an ecology capsule for themselves. Should they learn of the Metroids' capabilities, they could be used as a biological weapon against the Federation. Frantically, the Federation initiates a manhunt, and discovers a Pirate stronghold on a dwarf planet, Zebes. The capsule is thought to be at the center of the stronghold, where the massive computer "Mother Brain" resides.
Though they tried to penetrate this stronghold themselves, they did not have enough power to overcome any of the Space Pirate's weaknesses. They decide to entrust this mission to the Space Hunters, but with no ordinary bounty. Samus Aran accepts the task from the Federation directly.
The plot splits from here, as the remainder of the book is a series of choices that the reader can make. The reader can die at any point, including to small enemies during "fight" sequences (which occur in mostly any room you enter), and decide the order in which beams and other upgrades are acquired, just as in Metroid, as the reader can decide which direction they want to move. A lot of choices (particularly success in defeating an enemy) are determined via rolling a six-sided die.
It is revealed that the Metroid developing in the stolen capsule has broken out of it and mutated into a massive Metroid with pointed, tentacle-like arms, which Samus calls "Metroid=Mutant", or "M=M" for short. She can run into M=M at different points throughout the book, with the choice to fight or flee. Sometimes M=M will flee itself.
In addition, there is also a story arc after the player defeats Mother Brain. In it, Samus ends up being forced to dock inside a Space Pirate mothership, where she ultimately ended up unmasked and her true gender revealed for the first time. She then managed to encounter the Pirate Boss and entered a sword duel (depending on the player's choices, Samus could end up dying from being stabbed in the abdomen). Upon the Pirate Boss's defeat, she then evacuates the ship before it explodes. Several portions of this plotline, notably Samus being unmasked and her being forced to enter a Space Pirate Mothership after a failed escape from Zebes, as well as destroying the mothership after dueling with its (de-facto) commander, were later reused with Metroid: Zero Mission, a remake of Metroid.
The following translation is provided by GlitterBerri.
Tens of minutes passed as the pirate underlings struggled to remove my helmet. In spite of their difficulties, the lock eventually gave way.
As the pirates lifted my helmet, inch by inch, their mute amazement became ever more apparent.
...None of them had expected to witness the sight that was now before them.
For the first time, they were able to hear my voice.
"Yes. As you can see, I am a human woman." -> Proceed to 285.
Some of these images were previously sourced from Nicovideo (requires login). The video begins from the start of マザーブレイン撃破後の第二部, or After Destroying Mother Brain, Part 2, and includes some of the pages of possible choices the reader can make from there. The book itself is 285 pages long. Translations for the images can be found on their respective pages.