Shiori is the child of a rich family that gets killed in front of her eyes. She is the only one who survives because a devil saves her. She then decides to keep the devil by her side as a butler in order to be able to take revenge for her parents. Disguised as a boy, Shiori takes over her family business, 'The Fantom Corporation'. Under the cover of that corporation, she and her butler Sebastian start to search for the killer, which isn't as easy as Shiori might have thought.
When Shioris parents get killed in front of her eyes, she is saved by a devil and he starts to stay by her side as her butler. To take revenge, she goes back to her house as a boy, to inherit the Funtom corporation and searches for the killer. The devil beside her is always there to take her orders and does, whatever she tells him to. Until they can finally find the organisation who killed her parents.
If you want to watch this movie, I suggest you bring two things with you. First of all, you should NOT think about the manga while doing so. And secondly, you should like Hiro Mizushima. I did both things and to be honest, I was able to enjoy the movie pretty well! What people will be most interested in is probably, how much the movie differs from the manga... Well, it starts with the originally tiny boy being a girl. And then there are many characters missing. Of course they couldn't fit in a long storyline, so for the most part there are no other devils (Grell) and some other side characters are missing (the cook).
If you look at it as just a movie, that by chance has similar characters like the manga, the result is actually decent! Most of the time, it's there to show how awesome Sebastian is and to be honest, I love that because Sebastian is the reason I watched it. The rest of the movie is a tiny, obvious detective story. There are only two or three things I couldn't see directly, but most of the time I got the hint immediately and with that, I wondered what the movie wants to show us, beside Sebastian.
So, in the end, I can say I enjoyed the movie. Though there were some scenes that I thought were riddiculous, I still can give this movie 4* because it showed me exactly what I wanted. A warning to those who watch it for the sake of the manga: don't. And a clear recommendation to watch it, if you love Mizushima Hiro.