Taking place in the year 2027, Earth is suddenly attacked by the planetoid space fortress Zektron. Many cities are decimated and occupied by the Zektron forces, threatening to annihilate humanity. An underground military squadron and lab develops a small mecha called Dion (Imperium in the US version) aboard the mothership Leinoa, a mecha capable of harnessing great firepower. The human's goal is to send Dion/Imperium into Zektron and destroy it only after the Earth occupation forces have been eliminated.
Imperium differs from most shooters in many ways. There is no high score counter, but instead an experience points counter. Players earn experience points by destroying multitudes of enemies and the numbers differ depending on the size of the enemy. Players then earn additional weapons and power-ups for each weapon through a set number of experience points. The player has a five-hit life bar. If a player takes a hit, then whatever weapon they were using when they were hit is down graded by one point. Players can replenish health and any downgraded weapon by earning back experience points. The player also has a speed-change which they can change the speed of their mech to five different levels. The player has no lives. If the player dies, then it's Game Over. However, they can start on the level they died on by a Continue option that shows up on the main menu after dying. The player however only has four continues.
Although the game played out the same and featured similar music and sounds, there are a few differences between the East and West versions:
- Armored Maneuverer Dion featured large level design detail with different backgrounds layered over each other, particularly in level 2 where large construction beams forming squares span over the beach and ocean background. The extra layered backgrounds were removed in the US version Imperium.
- The Wave weapon in Imperium was originally a large fireball weapon in Armored Maneuverer Dion.fr:Kidō Sōkō Dion