Harry Potter and the Half-Blood Prince is the sixth book in the Harry Potter series which was written by J.K. Rowling.
As school starts, Voldemort and his Death Eaters are causing mayhem throughout the country and with Cornelius Fudge, Minister of Magic resigning over Lord Voldemort and his ways of not paying attention to the situation. Before he leaves power, he informs the muggle minister of the situation which is having different effects to the two worlds, magical and muggle worlds - but as this happens, Professor Snape makes a Unbreakable Vow to Draco Malfoy's mother, meaning he will keep Draco safe.
During his summer, Harry Potter helps persuad an old professor back to Hogwarts, before spending the remainder at his friend's home. During this time, they visit Diagon Alley and find Draco Malfoy at Knockturn alley - and while they follow him, they start suspection him to be a Death Eater.
Once back at Hogwarts, Prifessor Snape is annoucned as the new Defence against the Dark Arts teacher as the new Slughorn replaces Snape at Potions. As Harry and Ron prepare for their 'N.E.W.T.' exams, Harry is giving an old potions book which says this belonged to the Half-Blood prince - and while these notes help Harry during his work throughout class, the true identity remains a complete mystery.
Meanwhile the Death Eaters continue attacking the two worlds, which could be linked to Hogwarts. Meanwhile a student is badly injuried during a trip to Hogsmeade with a cursed necklace which could be linked with the Imperius curse. While the three's relationships takes different turns, as Harry discovers his feelings for Ginny, Ron's younger sister and Hermione with Victor Krum and Ron's realtionship with Lavender Brown - but not all realtionships are bound to last.
Meanwhile Professor Dumbledore secretly teaches Harry in which they use the Pensive to collect memories from Voldemorts past and Dumbledore reveals that Voldemort may have splintered his soul into six different fragments which are called Horcruxes which allow him to recieve immortality - with the seventh part in his actual body - but two have already been destroyed with Tom Riddles Diary - which Harry destroyed in his second year and when Professor Dumbledore destroying Marvolo Gaunts ring.
Harry and Draco Malfoy comes to blows when they duel but Snape comes to the rescue, in which he tries to retrieve the Half Blood-Prince's potions book but Harry may fool him. As Harry misses the Quidditch finals, he finds that Ginny has feelings for him to and the pair become a couple.
As Dumbledore assures Harry Malfoys behavour isn't worring, he maintains that he still trusts Snape - even when Harry confronts him about a secret leading upto his parents death. Meanwhile Dumbledore finds another Horcruxes and asks Harry to help find and destroy it. Before he leaves, Harry ask's his friends, Hermione, Ron and others to help keep an eye on Snape and Malfoy. While Dumbledore and Harry disapparate to a secret cave where they retrieve and locket which is also a Horcrux which makes Dumbledore weak after drinking a mysterious liquid which is needed to trieve the locket. Meanwhile Hogwarts come under attack.
When they leave the cave they find Voldemorts mark above Hogwarts - they fly to the Astronomy tower where they are ambushed by Draco Malfoy in which Dumbledore hides Harry under the imvisibility cloack and paralyses him just as Draco Malfoy disarms Dumbledore who admitts to being behind the death eaters attacks but Dumbledore refused to believe Draco isn't under Voldemorts spell.
Meanwhile members of the Order of the Pheonix arrive at the battle in which they battle Voldemorts death eaters in the castle. As two more death eater appear and tell Draco to kill Dumbledore but as Draco Malfoy hesistates, Snape appears and kills Dumbledore while Harry follows Snape and Draco out of the castle, the battle ensures. And as Harry is dueling with Snape, he reveals that he is in fact the half blood prince.
As the school year ends with Dumbledores funeral, everyone is in mouring and Hogwarts is rumored as closing, Harry ends his relationship and vows to find and destroy all the remaining horcruxes.