The inhabitants of the three worlds in the Alpha 1 Solar System have been at war for 7 months before hostilities upon the threat of invaders from the artificial world NGC-1611. The Alpha 1 Federation has gathered together the systems best defenses in an effort to repel the invasion. Three pilots and their machines are dispatched to NGC-1611, charged with the task of destroying the invader's power source and, if possible, locate the legendary weapon Xardion in the process.
Planet NGC-1611 is a typical world in all ways, omitting that it was constructed from raw materials by an unnamed scientist. His desire was to create a planet which could not die. His work was dubbed "Project New Frontier." The project was a failure, and violent monsters rapidly evolved, making NGC-1611 uninhabitable by humans. The scientist's final instructions to the supercomputers that once managed the planet were: "Protect my daughter! Eliminate all intruders."
However, the scientist's daughter passed away after these instructions were given. The supercomputers, not knowing how to proceed with their directive, eventually found a holographic image of the scientist's daughter and projected it into a protective capsule. They chose to defend that capsule in place of the actual girl. Sometime after, an undisclosed event or chain of events lead the supercomputers to send an invasion to the Alpha 1 system.
While possessing the typical qualities of action platformers at the time it was made (jump, duck, shoot etc.), Xardion made use of a few novel elements. Additionally, the game had a role-playing element where experience points could be accrued by defeating enemies and bosses, and each level up increased the player's maximum life and special ability points.
During the game, the player takes control of one of three available robot forms. Each form represents one of the three inhabited worlds in the system: Alcedes, from the Fire Planet; Triton, from the Water Planet, and Panthera, from the Desert Planet.
Each form has its own particular abilities and weaknesses. For example, Panthera is less than half the height of the others, and as such can enter narrow passages to collect items. Additionally, each form possesses its own health and special ability points, and must be upgraded separately from the others. This is equivalent to a 'tag team' game mechanic, where one heavily-damaged character can be swapped instantly for another.
Eventually, one of the robot forms is replaced by Xardion, which combines the abilities of all three previous forms and exceeds them. Xardion only becomes available near the end of the game, but joins the team at level one. An obstruction that can only be cleared by a sufficiently leveled Xardion forces the player to return to earlier levels in order to gain levels and develop special abilities.
Source: Wikipedia, "Xardion", available under the CC-BY-SA License.