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Narpas Sword is a famous password in Metroid that was purposefully built into the game and will not check with password generators. It is the only conventional "cheat code" to ever be included in a Metroid game.
All 0 characters are zero, except for the one O in sword. The eight final zeros are optional, and may be excluded.
When used, Samus will be given infinite energy and Missiles, an altered Ice Beam, and every power-up in the game with the exception of Energy Tanks and Missile Expansions (which are not needed anyway, considering the infinite amount of ammo and energy). Samus' energy cannot go below 30, or above 39. Although Samus will lose energy if she gets hit, she will gain energy if it falls below 30. Also, Samus can obtain a maximum of 6 Energy Tanks and 5 Missiles, though the Energy Tanks will never fill up and the Missiles will never increase or decrease (except when Samus defeats Kraid or Ridley, though it will revert back to 5 when she can move again).
Entering Tourian is possible straight away by using the Reo from the room right before the bridge room and freezing it to use as a stepping stone. There is a glitch in the programming when using the code: if Samus destroys a Metroid without obtaining a Missile Tank, and it attempts to drop a missile pickup at the last frame, the game will freeze. This glitch was not fixed in any of the later rereleases of Metroid (such as emulated versions included with Metroid Prime and Metroid: Zero Mission, as well as the Virtual Console rerelease), as Nintendo is apparently unaware of the code's existence.
It is interesting to note that when this password is used, the Ice Beam's projectiles are represented by the sprite for the Wave Beam projectiles, despite being colored like the Ice Beam and behaving exactly like the Ice Beam. This appears to be the result of trying to give Samus the Wave and Ice Beams simultaneously; obtaining the Wave Beam while using this password does nothing. The sprite is the same one used to make up the Wave Beam only rendered a different color. This combined sprite can be seen by selecting both the Ice Beam and the Wave Beam using a Metroid password editor.
There have been some debates on what the password stands for. Some players believe it refers to a "Narpas" sword, Narpa's Sword, or possibly even Narpas's Word. Others feel the password is properly read as "NAR Password", with several suggestions having been offered for the meaning of "NAR": an abbreviation for the name of the person who handled the conversion from the Famicom Disk System and designed the password system (Tohru Narihiro); an acronym for "North American Release" (unlikely since the password also works in the PAL version); or an acronym for "Not A Real", as in "not a real password." Another meaning could possibly be from the Japanese word "naruhodo," of which "naru" could be a shortened, slangish form which would be written as "nar" after being Romanized. "Nar password," to a native speaker of Japanese, would indicate a secret password along the same lines as the passwords needed during the era of prohibition to enter a speakeasy.
- If one beats Metroid with NARPAS SWORD equipped and presses start after the credits, they can play again as suitless Samus with the NARPAS SWORD benefits and both minibosses defeated, allowing for a quick path to Mother Brain and the best ending.