The basic idea is that this is a story about Severus Snape and someone of my own creation, Selene.
Selene is a Ravenclaw, should have Slytherin; Cunning, and ambitious, she only asked to be put in Slytherin so should wouldn't have to deal with others of her 'kind'. Late in her fifth year, she offers Severus help with his 'little' problem with the marauders. Leery of her intentions at first, Severus stood clear of her, oblivious to her attempts to help. Determined and persistent, Selene pranked the marauders over and over, yet they could not figure out who had done it. Eager to find a scapegoat, the marauders, blamed Severus, resulting in his detention and punishment. Severus cornered Selene, questioning her motives. Selene answers that she does not have any, she simply wishes to see the bullies served. Thus began their unlikely friendship.
In sixth year, Severus reveals that he plans on joining the Death Eaters. Although Selene is very against this idea, she understands that after Severus's experience at home and with Lily, that he is determined not to have to stand alone. Understanding his choice, she pledges to him that she will always be by his side, no matter his choice. Always. In turn, she asks him to postpone getting the mark for as long as possible. During winter break that year, Selene brings Severus back to her home. Her parents are filthy rich their investments in the magical real estate in Australia profiting immensely as the magical community flees there to escape the impending war. As an 'accidental' baby, Selene's parents act as though she doesn't exist. And although she may have free reign over their bank account, Selene is persistent on her belief that she must earn her own money, lest she become people like her parents. The neighbors come to visit, witches and wizard too pretentious to go to Hogwarts, and are homeschooled instead. During their exchange, Selene is ridiculed for her lack of style, out of style robes, and because their home schooling allows magic, they hex her, and she is unable to defend herself. Severus watches this exchange and it dawns on him her reasons for helping him, she too is rendered helpless by the differences in her societal class and her relations.
At the train station end of sixth year, Severus confronts Selene on a rumor that has been spread about Selene's 'relations' with Remus. (Fitting, as Selene is the Greek goddess of the moon.) Angered by his accusations, and his evident mistrust, Selene strikes him and angrily leaves the station. Taking her anger as a confirmation to the rumors, Severus returns home and immediately plans to receive the Dark Mark during the summer.
Seventh year, Selene stands outside the 'junior Death Eater's' train compartment until Severus agrees to talk to her. Severus has learned Legilimency and demands to perform it on Selene to know the truth. Humbled by his misunderstandings, Severus apologies and confesses that he has taken the mark. Despite her anger, Selene honors her promise never to leave him, and agrees not to make their friendship public as it would cause unwanted attention for the both of them. The continue to meet after classes in the come and go room and while their past meetings had been about planning pranks and scheming against the marauders, the attention has shifted to the movements of Death Eaters and keeping Severus alive.
After graduation, Selene starts a business by selling wooden animals that she has charmed to act as pets. Severus begins and apprenticeship, and continues his work with the Death Eaters. After he is forced to torture a red-headed muggle who reminds him a bit too much of Lily, he begins to regret his decisions and goes to Selene for guidance.
Selene enters the Order, offering her abilities as a 'seer' to help the war effort. She pretends to be an excellent military strategist, claiming the ability to know where when Voldemort will attack. In truth, she only translated information from Severus and decided what to tell the order, and what not to tell. So together, Severus and Selene played god, a spectators of game of chess in which the pieces moved on their own accord yet were bounded by the information and misinformation, given to them by unknown keepers. Together they helped the white pieces undercover the secret that the black king really had six souls and encouraged the pawns of the light to destroy these hidden hourcruxes, sacrificing their lives so the 'greater good'.
Before the 'checkmate' move was made, and the last time a dark lord would ever reign, Selene withdrew a large amount of money from her parents account and together with Severus, they disappeared from the magical community. Years after the war, the book "The See-er and the Snake" was published anonymously, and was read world wide by wizards and muggles, its unknown witer appraised for their 'creativity' and 'imagination'. But only two outsiders, and handful of those in the order with sharp enough minds to make the connections knew the truth: they were reading was not a story, they were reading history.