Before reading this book I was told it was 'seriously screwed up, but good.' Not a desciption too far wrong, but nonetheless could be elaborated on.
Confessions of a Mask is an autobiographical novel recounting Mishima's childhood upto about the age of twenty-five. He was born in 1925, so there is ample insight into Japanese life before and during the Second World War. Though Mishima was apathetic in regard to the war, he cared much about it from an individual point of view: for him the war represented the perfect chance to end his life, both heroically and alone.
His private fascinations with death, blood and gore pervade the book, and are inseparable from his suppressed homosexuality. Sexuality is the crux of this work; the book starts with Mishima's sexual awakening, and his surprise at its nature. From then on there is a divide between his private life, and his life in public, at school, with family. On his own he dreams of Saint Sebastian, his heroism and his martydom, yet with his friends he wears a mask, pretending an obsession for women. He takes this facade very seriously, and even engages in a catastrophic affair with one of his friends' sister.
The ending is indefinite. That is, one can't tell if he still maintains a barrier between privacy and publicity. However, throughout the book Mishima always claimed that facade would become reality; it is with the rejection of this theory, and the acceptance of his own sexuality, that the book ends.