"Gosenzosama Banbanzai!" is a Drama/Comedy OVA anime with some science-fiction elements, consisting of six episodes. Directed by known director Mamoru Oshii and produced by Studio Pierrot.
Basic plot: A small Yomota family, consisting of son Inumaru, father Kinekuni and mother Tamiko, live their normal life. Until a cute girl named Maroko Yomota from the future appears in the front door, claiming this family to be her ancestors, not at least Inumaru as her granddaughter. But the collapse of what has been a normal family is just the beginning of a chain of one mystery after another.
Having recently watched this anime not too many minutes ago, this has to be one of the funniest Mystery/Anime I've seen for a while. The animation is good, the drawing itself isn't so detailed as several other works by Mamoru Oshii like "Ghost in the Shell", "Patlabor: The Movie" and "Dallos". Some humor also appear in some scenes, especially in the opening "bird sequences" which appear before the first five episodes. This is what makes this small OVA series to be great in my opinion. So yes, Oshii knows how to make Mystery/Anime in general.
Are you into anime which have mystery and some humor in general, I suggest you to look for this OVA series once a time. My rating is 8/10.
