The war against Voldemort is not going well; even the Muggles have been affected. Dumbledore is absent from Hogwarts for long stretches of time, and the Order of the Phoenix has already suffered losses.

And yet… as with all wars, life goes on. Sixth-year students learn to Apparate. Teenagers flirt and fight and fall in love. Harry receives some extraordinary help in Potions from the mysterious Half-Blood Prince. And with Dumbledore's guidance, he seeks out the full, complex story of the boy who became Lord Voldemort — and thus finds what may be his only vulnerability.