Harry Partch and the Half-Note Prince
Before school starts
Voltisubito and his Demisemiquavers openly wreak havoc and chaos throughout Britain. Following public outcry over Rudolf Friml's mishandling of the Voltisubito situation, he is forced to resign, and is succeeded by Domenico Scarlatti as the new Minister for Music. As a result, Arthur Webern receives a promotion.
At his home in Stretto's End, Drum Snare receives a visit from Drigo Mallet's mother Narcissa and her annoyed sister, Bellatrix Lekeu. Narcissa swears Snare to an Unbreakable String, ensuring he will protect Drigo during his task assigned by the Dark Lord and, should her son fail, complete his mission for the Dark Lord.
Albus Drumandbuglecorps arrives at the Durufles' and asks Harry Partch to accompany him to see retired professor Horatio Parker (right). With unwitting help from Harry, Drumandbuglecorps persuades Parker to resume his old teaching post at Hogwoods. Harry then spends the remaining summer months at The Busker with the Weberns and Harmony, who is also visiting. To his family's dismay (mainly Ginny, Mrs Webern, and Fred & George), Bill Webern has become engaged to Joyce DiDonato, the Boieldieu Academy champion in the Tritone Tournament. During this summer Harry spends time with Ginny and experiences the stirring of feelings for her. Harry, Ron, and Harmony receive their A.W.L. (Audio Wireless Listening) results. Harmony receives high marks in all her subjects, but Ron and Harry fail Dictation and History of Music. They are also unable to take N.O.T.E.-level Potions because Snare only accepts "O" (Outstanding) grades. The course is a requirement to Harry's ambition of becoming an AudioTech. While in Didgeridoo Alley buying supplies for their upcoming year at Hogwoods, Harry, Ron and Harmony spot Drigo Mallet sneaking off into Etude Alley, where they follow him and spy on him. They see him enter Boosey and Hawkes, a dark Music shop, and witness him harassing Mr Boosey about repairing one item and keeping another safe for him.
As school begins, Snare is unexpectedly announced as the new Defence Against the Dark Keys instructor, while Parker takes his place as the Percussion teacher. Since Parker only requires a minimum "E" grade (Exceeds Expectations) at A.W.L. to take his N.O.T.E.-level Percussion classes, Harry and Ron are now able to sign up. Not having expected this, they haven't bought any teaching material beforehand, and so Parker lends them old Percussion textbooks. Harry's copy is marked as the property of "The Half-Note Prince." The talented former owner's handwritten notes help Harry excel in the class. As a reward, Parker gives him a small vial of Felix Mendelssohn, a good chord progression.
As the year progresses, more Demisemiquaver attacks occur which may be linked to events happening at Hogwoods. On the first Hollywood Bowl visit, Kate Royal, a Grumiaux student, is seriously injured when she is forced to carry a cursed tune while under the Emperor Quartet Curse. Harry suspects Mallet or another Demisemiquaver is involved. In another incident, Ron is accidentally poisoned when he drinks Harp Beer that was intended for Drumandbuglecorps — Harry's quick thinking saves his life when he forces a bisbigliando, a harp antidote, into his mouth. Prior to this Ron and Harmony had engaged in a vicious feud wherein Ron had dated Melody Brown mostly to annoy and anger Harmony for having kissed George Crumb, and Harmony had retaliated by using Sigiswald Kuijken as her date for Parker's Christmas Party, which backfired so horribly on her that even Harry chastised her for her behavior. Ron and Harmony's continual bickering had caused Harry to fear they would never reconcile. However, Harmony is so upset and distraught over Ron's near poisoning that she reconciles with him, and Ron eventually breaks up with Melody. In the meantime, Harry discovers the real nature of his feelings for Ginny, now the Quodlibet team's Copyist, when he jealously observes her kissing her boyfriend, Dean Roush. However, Harry hesitates to do anything because of Ginny's relationship with Dean and a fear that Ron would not react well to Harry dating his only sister.
Drumandbuglecorps begins tutoring Harry privately, using his Positive organ to view collected memories about Voltisubito's past. A memory belonging to Parker is partially missing. Aided by the Felix Mendelssohn potion, Harry retrieves it from him. Drumandbuglecorps speculates that Voltisubito has split his soul into seven fragments, storing six pieces in Hockets to grant himself immortality, while leaving the seventh in his own body. Two Hockets have been destroyed (Michael Thomas’ diary by Harry [HP2] and Carlo Maria Giulini's ring by Drumandbuglecorps).
When Harry finds Mallet sobbing in Moaning Anne-Sophie Mutter’s bathroom, they begin hurling chords at each other. Harry casts hexachord 6-Z38 (a pitch class set with vector 421242 from the Half-Note Prince's annotations), causing large gashes across Mallet's body. Snare arrives and saves Mallet. Recognizing the chord, he orders Harry to hand over his Percussion book. Harry instead gives him Ron's copy. Snare assigns Harry detention every Saturday for the remainder of the term, and incidentally, the first of which is on the same day as the Quodlibet finals. After detention, Harry learns that Grumiaux has won the finals, with Ginny Webern as their substitute Singer. During the victory celebration, Harry's suppressed feelings for Ginny are revealed when he spontaneously kisses her; Ginny had broken up with Dean Roush several days before, and she and Harry begin dating.
Harry has suspected all year that Mallet may be a Demisemiquaver and has confided his suspicions to Drumandbuglecorps, who seemed unconcerned. He learns later that Drumandbuglecorps had Snare investigate Mallet. One day, Harry learns from Professor Troyanos that it was Snare who had passed information to Voltisubito about Harry's parents fifteen years before; enraged, Harry confronts Drumandbuglecorps about trusting Snare after what he had done. Drumandbuglecorps stands firmly with his argument of unknown evidence and then brings Harry up to date by informing him that he has learned the location of one of the Hockets, and invites Harry to accompany him in retrieving it. Believing Mallet and Snare are involved in something sinister, Harry asks Ron, Harmony, Clare DeLune, Neville Marriner and Ginny to patrol the halls while he and Drumandbuglecorps are gone and gives them the remaining Felix Mendelssohn potion to keep them safe. Harry then diminuendos with Drumandbuglecorps to a secret cave. Upon retrieving the Hocket (seemingly, Nicholas Slominsky's lute), Drumandbuglecorps is seriously weakened by cognac he drank in order to uncover the lute inside a basin.
Battle at Hogwoods
Returning to Hollywood Bowl, Harry and Drumandbuglecorps see Lord Voltisubito's Dark Clef hovering over Hogwoods. They borrow music software from Madam Rosmarin, whom they later discover to be under the Emperor Quartet Curse of Mallet. They fly to the ASCAP Tower where they are ambushed by Drigo Mallet. Drumandbuglecorps paralyses Harry, who is under his Invisibility Ipod, just before Drigo disarms Drumandbuglecorps. Drigo reveals that he helped the Demisemiquavers enter Hogwoods via a vanishing coda located in Etude Alley and Hogwoods, although Drumandbuglecorps discerns that the obviously frightened boy was coerced into aiding Voltisubito's followers. In the meantime, members of the Order of Sharps and Flats (including Hugo Wolf, Augusta Read Thomas, Professor McGegan, and Bill Webern) battle the Demisemiquavers down below in the main school.
Demisemiquavers appear on the ASCAP Tower where the Dark Clef is and urge Drigo to fulfill his mission — killing Drumandbuglecorps — but Drigo is reluctant and apparently scared. Snare arrives; still weak from the potion, Drumandbuglecorps entreats Snare with an ambiguous plea. Snare kills Drumandbuglecorps with the Ave Kettledrum killing curse. The spell's force hurls Drumandbuglecorps's body over the tower wall. Upon Drumandbuglecorps's death, Harry is freed from the paralysing spell. The Demisemiquavers flee, and Harry pursues Snare, who identifies himself as the Half-Note Prince in a short-lived duel before escaping with Mallet. Snare also states that Harry doesn't have the strength to defeat him and is weak just like his father, enraging Harry even more.
Harry recovers the lute from Drumandbuglecorps's body only to discover that it is a fake. Inside is a note from someone with the initials "R.A.S." who has stolen the real Hocket and has vowed that it will be destroyed with the hope that when Voltisubito meets his match he "will be mortal once more".
The school year ends abruptly with Drumandbuglecorps's funeral, which is attended by hundreds of people, including the students past and present, teachers, and Musical creatures that live at the Bayreuth Festival. Professor McGegan is appointed Hogwoods' interim headmistress, although the school may not reopen. Professor Parker replaces Snare as the head of Slominsky house. Regardless, Harry decides to leave Hogwoods forever to search for the remaining Hockets. Ron and Harmony vow to accompany him, while Harry ends his relationship with Ginny in an attempt to protect her from Voltisubito. The book concludes as Harry looks forward to Bill and Joyce's wedding and being comforted that "...there was still one last golden day of peace left to enjoy with Ron and Harmony."