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.