Gradius is the NES version of the arcade game of the same name. It's an horizontal shoot-em'up and the first in the Gradius series.
The planet Gradius is invaded by the Bacterion Empire, the trans-dimensional starfighter "Vic-Viper" (Named "Warp Rattler" in the American localization..) is send on a counter-attack to destroy the "Xaerous Space-Fortress.", home of the invader.
A unique feature of Gradius is the power-up system. In the bottom of the screen, there is a bar with the multiple amelioration, collecting an capsule (which can be gained by either destroying a formation of enemy or destroying orange-colored enemy) will highlight the first option: Speed Up, which boost the speed of the ship. The player can either choose to activate it, reverting the Power-up bar to its original, non-highlighted state, or picking-up another capsule and spend it on the next option: a missile. The power-up are: Speed Up, Missile, "Double" (A second shot at a 45-degree angle.), an armor-piercing laser, two "Option" (Drone that mimic the player movement and firepower.) and a shield. Double and Laser can't be activated at the same time, it's either one or the other. Another type of power-up is the blue capsule, which destroy every weak-enemy and shot on the screen.