It basically involved some timey-wimey craziness with a version of Eight late in his time-stream -it's implied that it's when he's fighting in the Time War, but not outright stated- and another version of him from early in his timestream, predating the events of "Storm Warning". A lot of the story shows where Mary got some of her inspiration for Frankenstein, including the name. The future Eight also dropping more cryptic hints as to what kinds of adventures he and Mary had together. For example, he mentions an encounter with the Cybermen, but more on that later. After the crisis has been resolved, the younger Eight offers Mary a chance to travel with him. She replies "Your future self told me I had traveled with him, and it seems it's my destiny. It appears I have no choice in the matter." But Eight assures her there's always a choice, and that she can stay safe at home with her husband if that's she really wants.
Mary decides to accept Eight's chance to travel through time and space, but she asks how she should address him. Eight replies "Just 'the Doctor' will be fine." When she asks about the surname Frankenstein, he replies "You have to remember, it's very important: Frankenstein is the name of the monster, and not the name of the Doctor." And so they're off. So, that was how they met, but what about all those adventures? Well last year, they released a trilogy of stories with Eight and Mary Shelley, detailing some of their adventures together.
There's been some grisly murders happening, and someone is using a Cyberman for entertainment purposes. It's pretty gripping, and I especially liked how it tied into Marc Platt's other Cyberman story "Spare Parts" with Eight telling Mary how he witnessed their creation on Mondas. It's little details like that which make me enjoy it that much more.
There's been a bit of a theme throughout these 3 stories that show where Mary got her inspiration for many of the story elements present in "Frankenstein". That's all well and good, but I'd also love to see a story where Mary encounters something she can't write about. Whether it's because Eight forbids it or because she's too terrified that anyone would believe her, it would be a great and intriguing way to explore her in a fresh new way that not many people get to explore a historical figure in a fictional format.
However, The fourth part of "Army of Death" has a little stinger after the ending music where Mary is writing in her journal that she wants Eight to return her home. If this means that this is the end of Mary's travels, I don't know. But I'm really hoping this isn't it. I remember the older Eight in "Mary's Story" mentioning an encounter with the Axons, and I'm really intrigued how that turned out. Big Finish, make it happen! More Eight and Mary Shelley, please! If anything, it'll give me more to rave about on this blog!