Henry is a boy without many exceptional talents, or so he believes. However, his mother thinks he might have a talent for magic, so she ships him off to the Wizard's Hall to learn the ways of sorcery. There, he is given the name Thornmallow—"prickly on the outside, squishy within"—and dumped into magical classes. Thornmallow proves to be as lacking in magical abilities as he is in other things—except for one odd excluded incident in which he causes an avalanche of snow in his classroom, with a single rose growing vibrantly in the center.
But Thornmallow does have a (smudgy) nose for trouble, and he's smart enough to notice that not all is right at Wizard's Hall. The Magisters seem to be frightened. The supposedly lush gardens of the Hall are dead as doornails. He and his friends—good-natured Willoweed, energetic Tansy, and impish Gorse—start to do research into the matter, and discover a prophecy: The evil wizard Nettle, and his horrible Quilted Beast, are coming to claim Wizard's Hall, and only the 113th student can stop him. Thornmallow is that student. But he's a terrible wizard! What will become of Wizard's Hall?
This book contains examples of:
- Accidental Misnaming: Nobody can seem to get Thornmallow's name right.
- Arc Number: 113.
- Baleful Polymorph: Dr. Mo, a rodent-looking thing in a cage that can only say "Squark!" turns out to be [[spoiler:Magister Dr. Morning Glory, shrunken and cursed by The Master.
- Big Bad: Nettle.
- Bittersweet Ending: Thornmallow will never be a great wizard, and just being around wizards will cause him pain for a long time, because of Nettle's stings. But He Chooses To Stay anyway, because he wants to be with his friends and do what he can anyway.
- Cursed with Awesome: Thornmallow turns out to be an enhancer - that is, he can amplify someone else's magic terrifically. It doesn't particularly help him cast spells on his own though.
- Dark Is Not Evil: Thornmallow is corrected that the "black arts" are not evil, the term being from the fact that, like the bottom of a lake, they are deep and dangerous.
- Disappeared Dad: Thornmallow's father is only mentioned once, and never seen. It's unclear if he's even alive or not.
- Dreadful Musician: Thornmallow is tone-deaf, which is a serious handicap in a setting where magic spells are sung.
- Eccentric Mentor: Really, eccentricity seems to be required to be a magister. Or maybe it's the magic itself that's eccentric and the magisters have to become eccentric themselves to master it.
- Fisher King: In a variant, even though Nettle isn't actually in charge of Wizard's Hall, as long as he's out there, no plants grow in the Hall's garden.
- Foreign Queasine: Wizards eat Lizard Soup for dinner. It tastes like mud. Learning to transform it into something that's actually good to eat is part of the Wizard's Hall curriculum. Unfortunately for Thornmallow, First Years can only change the appearance, not the taste.
- I Choose to Stay: Thornmallow has the opportunity to leave Wizard's Hall at the end, but decides to stay anyway.
- The Main Characters Do Everything: Averted. A girl who wasn't anything besides a face in the crowd plays a significant and important part in one of the final scenes.
- Meaningful Name: All wizards get them. They're usually plant-related. Thornmallow's real name is Henry, but the Magisters say it's just a bunch of nonsense syllables. They pick Thornmallow because he's 'prickly on the outside, squishy on the inside.'
- Mystical White Hair: Thornmallow has hair "as white as dandelion fluff".
- Nothing Is Scarier: Discussed. "Which is worse—seeing or not seeing?"
- Running Gag: None of the Magisters can get Thornmallow's name right.
- Ship Tease: Between Tansy and Thornmallow. They hold hands a couple times, and Thorny finds her comforting.
- Tomboy: Gorse takes great pride in her prickly name and boyish attitude. She also likes grossing people out.
- The Unintelligible: Dr. Mo, who can't say anything besides "Squark!"
- Wizarding School: Wizard's Hall is a school where children go to learn wizardry. Those with no magical aptitude are either turned away at the gate or unable to make it in the first place due to the enchanted road leading to it.
- Words Can Break My Bones: And blind you, and deafen you, and make you wracked with endless pain... Even kill you.