Onegai Sensei is an anime about an 18 year old boy with a serious illness that will put him in a "stand still" or near death, who meats an alien who has came to earth to observe us and try to prevent us from going down a "destructive" path.
After the alien( mizuho Kazami) arrives at ours planet she gets seen by the boy(Kei Kusanagi ) and and they get caught stuck in a storage room. so to get out of this mess Kei and Mizuho must lie to her boss and tell him that they are married. and that night they got all of the papers done and they were officially married. after they get married a lot goes on. you will have to watch the anime if you want to fine out what happens.