One of the defining characteristics of the Gradius series is the use of a "power meter. " The power meter is powered by a power-up item, whose purpose, when collected, is to move a highlight to the next power-up on the power meter. When a power-up that the player wants becomes highlighted, he or she may activate it, also causing the highlight to reset.

Gradius, our mother planet. Bacterian, a cluster of stars located in subspace, has invaded this dazzling planet and exposed it to a series of vicious battles. Threatened with extinction in the face of such an onslaught, the planet Gradius decides to deploy its final hope: the super-dimensional fighter Vic Viper. Its destination is the enemy fortress of Xaerous. The only way to put an end to this conflict is to plunge deep into the enemy's stronghold and destroy the core that lies at its center. Prepare to launch! And so, Vic Viper takes off into the vast blackness of space.