In The Guardian Legend, the player controls the female guardian of Earth, a "highly sophisticated aerobot transformer". The player's mission is to infiltrate Naju, a large planet-like object which aliens sent hurtling towards the Earth. While inside, the player must activate ten safety devices in order to initialize Naju's self-destruct mechanism and destroy the alien world before it reaches Earth. Five hostile tribes of alien lifeforms are vie for control of territories within Naju, and the player needs to fight through them to successfully activate the switches and escape. The story is advanced through a series of messages left by one or more unidentified predecessor(s) who unsuccessfully attempted to engage the self-destruct mechanism of Naju before the Guardian arrived. Left by the sole remaining survivor of the attack on Naju, the first message serves as an introduction; later messages give hints that help the player open locked corridors.
Source: Wikipedia, The Guardian Legend , available under the CC-BY-SA License.