An alien soldier must break all the rules to protect humanity from self-destruction or lose the human woman he loves, forever.

He is on a secret mission on earth for twenty years, but it will be to complicated to explain.

I like it but perhaps explain a little why the soldier is there in the first place: Is he by himself?, did he crash-land? etc... I really like the concept though. How about:
"When an alien soldier develops a bond with a human women in the middle of an invasion of Earth, he will have to break all of the rules to save humanity and the women he loves."
What do yeah think?