The protagonist of the story, Rin, runs a consulting agency with her partner (insert double entendre) Mimi. Rin is the cool and collected field agent, while Mimi is the playful flirtatious girl who helps out by being a computer hacker. Through the course of the series, there are a number of other important characters they meet and work with that you won't be able to keep track of because after the second episode the show tends to skips years or decades between each. While it is somewhat interesting to see the progression of technology across decades, it serves little purpose except to allow certain characters to age and have children in a show with limited run time. Unfortunately, this means the viewers have little time to get attached to a character before they are killed off, so we can't empathize with Rin and Mimi's loss. Did I mention that Rin and Mimi are immortal? Well they are, and they don't age, so they will remain hot and young so we aren't grossed out when they have lots of lesbian sex (often with other immortal lesbians).
There is an overarching plot to the story, but it is presented only very gradually until the final episode suddenly springs the whole thing at us. In the meanwhile, Rin and Mimi, sometimes with the help of other people they know whom we can't remember if they have been introduced, solve puzzling cases that are obtuse even in their resolution. Not that the final outcome of the individual cases matter at all to the larger picture, since the tangential link between them tends to be the involvement of a particular family (genetic, not criminal), and that another immortal often appears in order to kill Rin for reasons that are never explained, although there are repeated mentions of someone named Apos who seems to be manipulating something and being cruel and sadistic about it.
Visually, the series is very dark, probably to convey the omnipresent brooding and ominous atmosphere of the series. For an OVA from 2008, the animation and art is good but unspectacular. Voice acting is generally well done, and the opening and closing are appropriately gritty, if not particularly memorable.
Interestingly, while Mnemosyne is a reference to the mythological Greek river of memory, the impetus behind the story is Yggdrasil, which is the world tree from Norse mythology.
