Memoirs Found in a Bathtub starts with the finding of a diary in the distant future. The introduction dwells on the difficulties of historical research on the fictional 'Neogene Era', "the period of the heyday of the pre-Chaotic culture, which preceded the Great Decomposition". "Great Decomposition" refers to the apocalyptic event of "papyrolysis", decomposition of all paper on the planet in the pre-information-technology era, causing all records and money to turn into dust––the end of the "epoch of papycracy".

The diary, known as the 'Notes of a Man from the Neogene', was found in the lava-filled ruins of Third Pentagon within the territory of the disappeared state of Ammer-Ka. Previously, little was known about the hypothetical 'Last Pentagon'. One researcher suggested that Pentagon was a kind of military brain, the center in charge of maintaining the faith of Cap-i-Taal, dominant in Ammer-Ka in the period of U-S. This was confirmed by the finding of the diary, supposedly of an agent trapped deep within the subterranean bowels of the vast Third Pentagon, although the authenticity and authorship of the document were questioned by some researchers.

The rest of the book is the diary itself. In a Kafkaesque maelstrom of terrifying bureaucratic confusion and utter insanity, the agent attempts to follow his mission directives, conducting on-the-spot investigations: "Verify. Search. Destroy. Incite. Inform. Over and out. On the nth day nth hour sector n subsector n rendezvous with N." The narrator inhabits a paranoid dystopia where nothing is as it seems, chaos seems to rule all events, and everyone is deeply suspicious of everyone else.