In Kagi-tsuki Terrarium, the entire known humanity lives in the arcology, a technologically advanced megastructure completely isolated from the outside world. An arcology is actually a real concept of mostly self sufficient structures that some people have conceptualised. In the manga's arcology, the frontier between humans and robots has become vague. Robots have human-like personalities, which are transferred to newborn humans. A few centuries before, a great war took place in the arcology. One of the results was the loss of most historical data of this period and prior to this. Since then, people have been living in several colonies throughout the arcology, oblivious to their past and even the mere existence of an outside world. Recently, a decrease of energy, automatically generated from lost technology, has been observed and technologists, people specialised in these technologies, have been sent to investigate.
Chico, a young technologist, is accompanied by Pino, her robotic little brother. Together, they follow the path of their mother, also a technologist, who disappeared a few years before as she was searching for data to create a universal key, able to control the systems of the arcology. Among the two main protagonists, Chico is the hot-headed and benevolent one, who tries and helps everyone in need, even if this may be detrimental to their mission and her health, while Pino is the composed one who tempers her sisters' impulses and protects her when needed. Together, they go from one colony to another, some dead, some on the brink of disappearing, some still thriving, and meet different people, humans and robots.
Without any spoiler, Chico and Pino will learn some truths about the arcology, humans, robots and their past, potentially questioning their beliefs. Of course, both of them will learn more about about themselves, leaving to some drastic decisions.
In the first three volumes, Kagi-tsuki Terrarium is basically a manga about two people travelling through a world that is slowly dying, while the last volume is more about the past of the arcology and its people. While it is neither subtle nor deep, the manga tackles various subjects, such as humanity, the difference between humans and advanced AIs, civilisation decline, as well as transhumanism and the ethical issues surrounding it. In the end, these four volumes are a nice little trip that does not overextend itself.
