Though the war against Voldemort is not going well, life at Hogwarts goes on, with the sixth-year students continuing their wizardly studies and trying to survive teen romantic dramas, while Harry himself probes ever more deeply into Voldemort's complex history.

Lord Voldemort tightens his grip on both the Muggle and wizarding worlds, leaving Hogwarts no longer a safe haven. Harry suspects perils may even lie within the castle, but Dumbledore is more intent upon preparing him for the final battle that is fast approaching. Together they work to find the key that will unlock Voldemort's defenses. To this end, Dumbledore recruits his old friend and colleague Horace Slughorn, who he believes holds crucial information....