Gradius: The Interstellar Assault is the second portable game in the Gradius series. It was released in 1991 in Japan as Nemesis II: Return of the Hero. It is also a sequel to the Game Boy game, Nemesis. Both games can be considered a spin-off of the main series, but unlike Salamander they don't appear to be part of the official Gradius canon seen in Gradius Galaxies.
Gradius: The Interstellar Assault retains the traditional horizontal scrolling gameplay from the Gradius series. Once again the player takes control of the Vic Viper and flies through five different stages destroying Bacterion's army.
The game retains the traditional power-up bar from the original Gradius. The player can speed-up multiple times, use missiles, shoot double firepower or lasers, use several options at a time and use the classic shield (although it is referred to as a forcefield). However, before each game is started or continued, the player is given a "Weapon Select" screen. Here the player can choose between one of three settings for the missiles, double firepower, and lasers.