"Your seat remains vacant. You won't finish your degree, you won't begin a postgraduate thesis. You will study no more".
He ("you") is a twenty-five year old student, living in Paris, suddenly overcome by the pointlessness of it all: "You have hardly started living, and yet all is said, all is done." He decides to withdraw from life, simply not accepting the inevitable, mundane path that has been mapped-out for him -- indeed, not accepting any future. It begins when he sleeps through an exam, the first step to his abandoning his studies
He doesn't answer the door or see his friends, cutting himself off from his old life. He simply drifts, realizing that: "you don't know how to live, that you will never know." He doesn't seem to want to learn, either, simply allowing himself to drift.
He stays with his parents in the country for a few months, lazing away. He returns to Paris, to his garret: "the centre of the world" -- his limited world, certainly. Almost: the entirety of his world.
He practically sleepwalks through life. He wanders about Paris, goes to the cinema. He reads every word of the newspaper, but the content is essentially meaningless to him and his life. He tries to impose an order on his life, but that makes little difference.
He grows no wiser.
He finds, in the end, also that: "Indifference is futile."
Perec closes the book with some hope of finding one's place in the world -- and as part of the world --, in his own way. His character is, ultimately, not an island. At the end he is waking, gently, just.