Harry Potter and the Half-Blood Prince
The book starts at Severus Snape's house. Narcissa Malfoy is very unhappy that Voldemort has made her son, Draco, put on a mission. Voldemort is a very bad man who does not like half-blood or Muggle (non-magical) wizards and witches. He wants to kill them. Bellatrix, Narcissa's sister, and Narcissa go to Snape's house. There, Snape agrees and says he will help Malfoy complete whatever his mission is.