The protagonist, Hodai, is struck with an arrow from Kurona, an evil angel. This makes every girl at his academy furious with him. As such, they run at him, attacking Hodai at the first chance he gets. Hodai must defend himself by shooting girls with a magic arrow, which makes them fall in love with him. The curse does not stop here though; if Hodai is unable to find his one true love by the end of the day, he is destined for a life alone. Hodai must protect himself throughout the adventure, while also seeking the love of his life.
Source: Wikipedia, "Gal*Gun: Double Peace", available under the CC-BY-SA License.