As implied in the title, I strongly recommend watching a couple, better yet all of the episodes of the TV series before this movie. Or, of course, read as many episodes of the manga as possible, but the TV series will be easier to acquire. The OVA might also help, of course, especially if you don't want to invest a couple of hours of "preperation" for the movie.
Why? First of all, obviously for chronological reasons, because the events in the movie occur ca. two years after the series ended. But more important is the fact that you will lack important background knowledge if you jump into the universe of "Ah! My Goddess" with the movie. For example, you would simply have to accept the fact that a Japanese student lives in a buddhist temple together with three goddesses without further explanation, among many other details (calling heaven via telephone - huh?).
One of the strong concepts of the series is that the key characters are slowly and sequentially introduced, allowing the viewer to grow on them. While it might not be necessary to actually watch all episodes of the series or the OVA, I think it is mandatory to watch at least the first ones until Urd and Skuld are introduced. The more, the better...
You will get sooo much more out of the movie if you don't have to wonder why the goddesses live on earth under rather mundane circumstances. Watching the romantic relationship between Keiichi and Belledandy being endangered will touch you far deeper if you are already acquainted to the AMG universe and actually watched it grow and blossom in the first place.
Imagine watching this latest Star Trek Movie with the young crew (e.g. Kirk and Spock being rivals in a time prior to their deep friendship) without knowing the series or earlier movies. You might enjoy the movie anyway, but will get only a fraction of the experience out of it compared to a trekkie.
I hope you get what I mean - watch the series first! :)
So far soo good. Well, what will you get out of the movie if you know the manga and/or the anime? In a few words: more than you would have expected! But let's first comment one some missing elements.
One would be the lack of near-nudity and (sexually) emberassing situations which would frequently occur in the series as comedic elements. No big deal - I loved them in the series, yet didn't miss them at all in the movie. Notable though, since they were one of the key elements in the series. I guess it's better this way, because the movie can be presented to a broader viewer spectrum.
Also I was a bit surprised that some key characters from the series did not appear at all in the movie (Marller, Hild and Sayoko). A new villain is introduced instead, and in a very clever way which makes it impossible to apply black/white thinking.
A refreshing element of the plot is that while in the series Keiichi ended up with some naked women, goddesses, even the devil himself in bed every other day, accidentally drank love potions etc, I cannot recall any episode where it was the other way round - and in the movie we actually get exactly this scenario. It is the first time (at least the first I can think of) that Keiichi gets a serious rival.
Furthermore, it is the first time since the invasion of the bugs at the very beginning of the series that the AMG universe has a really big problem and the future of the whole world is at stake.
There is not really much more to say... If you liked the series or the manga there is no doubt that you will love the movie. It is a worthy sequel for the series (as well as a worthy representative for the manga). The animation is beautiful and elaborate, the soundtrack is awesome. If you have a soft, romantic spot, prepare for it to be touched. If you don't, maybe you can enjoy the great artwork, but probably you will find most of the scenes a bit cheesy.
