The main character and his friends are attending their class in a quiet academy like normal students when a UFO suddenly passes overhead. The whole academy begins to panic and a beautiful, female alien appears from the UFO. She claims she traversed vast reaches of space in order to tie the knot with Yuusaku. Meanwhile, Yuusaku is utterly confused. As she tries to press him into marriage, she uncovers his secret stash of porn and begins using knowledge of cosplay from that in order to seduce him.
The other heroine, his childhood friend, joins in the fuss over Yuusaku caused by the alien's mother, friend, rival, and even space pirates. On top of all this, secret agents from another country begin targeting the Aliens' advanced technology...