Stargunner is a horizontal shooter released in 1996 by Apogee Software (now 3D Realms).
There are two storylines - one from manual and one from official site - but they dont differ much, except galaxy and race names. As usual in this type of games your planet has been attacked by enemy fleet and you must destroy it, together with final boss.
Stargunner features over 30 levels, many bonuses, powerups and a store where you can upgrade your ship.
Game was released as freeware in 2005.