Sherlock did not trouble himself to knock on the door to the Greenhouse. He barged right in, and I followed behind. It was a pleasant contrast within the Greenhouse to the wintry chill without, and I loosened the buttons on my coat.

Sherlock lingered at the door, inspecting both sides of it.

"I understand why a Greenhouse would have a lock on the outside," he said, speaking slowly, contemplatively. "Why would it also have one on the inside?"

Professor Dawlish was seated at a desk at the head of the room, its surface topped with a miniature forest of exotic herbs and potted plants. He was a conventionally handsome man just shy of six feet tall, with thick brown hair which was carefully styled.

He did not seem perturbed by our intrusion, nor did he deign to look up as he responded to Sherlock's inquiry. He was busily grading papers, feathered quill scribbling red ink on a hapless student's homework.

"It's been that way for ages, Mr. Holmes. Long before I took up my post — before I was even a student here."

"That doesn't answer my question."

Dawlish looked up, giving a thin smile. He beckoned for us to join him, and with a careless swish of his wand summoned a pair of stool chairs opposite him. We settled into them.

"If you must know," he said, "decades ago there was an Herbology Professor by the name of Herman Greentree —"

"—perfect name for an Herbologist—" I muttered.

"—and he had an obsession with magical flora which I daresay bordered on pathological. He would sleep here most nights, sometimes lying in the soil, curled up with plants like lovers. At some point he installed a deadbolt lock on the inside for his own privacy and security. All of my N.E.W.T. students have a key so that they can access the Greenhouse at any hour."

"Couldn't a wizard easily disengage the deadbolt with magic?" I said. "Aren't there — erm — spells to unlock doors?"

"Naturally," Dawlish said. "There are a number of Unlocking Charms available to a determined witch or wizard. But Professor Greentree deliberately used a special lock constructed of an enchanted iron ore resistant to magical tampering."

"Just how resistant?" asked Sherlock. "Impregnable?"

"No level of security is ever impregnable. There are at least a handful of witches and wizards here at Hogwarts powerful enough to disengage or bypass the deadbolt."

"Right," Sherlock said. "What is this?"

He was pointing at an orchid which occupied the corner of Dawlish's desk. It had orange stems and blue leaves.

"Dragonsbane," said Professor Dawlish. "Just a seedling. I discovered it recently during a trip in Tajikistan. Incredibly rare and quite valuable." He tickled the plant's leaves, and it wiggled contentedly.

Sherlock leaned over and whispered in my ear. "This plant was not in Hermione's memory."

Dawlish eyed us skeptically.

"I must admit, Mr. Holmes," Dawlish continued, "I share Professor Malfoy's apprehensions. You may be a brilliant detective and a master logician, you may be acclaimed and celebrated for your deductive feats by Muggles. But you are ignorant about magic — through no fault of your own — and thus ill-suited to the task bestowed upon you. I fear you cannot solve Dexter's murder, and your involvement in this case will only delay or complicate efforts to bring his killer to justice."

"Thank you," Sherlock said wryly, "for the rousing vote of confidence. We have some questions for you — you being a person of interest in the case, after all."

"Are you suggesting I'm a suspect?"

"No. Merely an exceedingly interesting person of interest. After all, it was here in your Greenhouse where Dexter was murdered, or at least where you discovered his corpse. And there is a possibility, however faint, that the Carnivorous Tentacula is what killed him, a plant which a student was cultivating under your tutelage."

"A mistake." Dawlish sighed, shaking his head wearily. "I admit it was a terrible mistake."

"What was?" I asked.

"The Tentacula which was found on Dexter's body — that plant was being cultivated by Scorpius Malfoy. He's one of my N.E.W.T. students, and I rather foolishly permitted him to select a lethal, venomous plant for his end-of-term project."

"N.E.W.T.?" Sherlock asked.

"Nastily Exhausting Wizarding Test. Scorpius was one of my 7th year students. He's a gifted herbologist. A gifted student altogether, based on what I hear from colleagues. He had a very bright future ahead of him, until… well…"

"Until what?"

Dawlish gave us a quizzical look. "You don't know?"

I shook my head. Sherlock remained still, his pale face impassive.

"Ms. Granger didn't tell us much," I explained. "She preferred we investigate and uncover clues independently."

Dawlish stroked his chin, a slight smirk on his face. He was seemingly amused by this admission.

"Very well. Then allow me to uncover a possibly very important clue. Dexter Zabini was a 7th year Slytherin Prefect. A couple weeks ago he apprehended Scorpius Malfoy performing an Unbreakable Vow on a first year student."

"Scorpius Malfoy," Sherlock said, "I assume he shares a relation with Professor Draco Malfoy?"

"Father and son."

"Wait — when we were coming here in the flying car Hermione placed us under an Unbreakable Vow. Why would —?"

"Binding a minor to an Unbreakable Vow is a major offense," Sherlock explained. "I presume Scorpius was past the age of adulthood?"

Dawlish nodded. "Seventeen years. Now listen. If any other student had done this, they would have been summarily expelled. There would have been a criminal referral. But Draco Malfoy… well — I'm not one to cast aspersions… and it would be impolitic for me to speculate…"

Dawlish looked like he wanted nothing more than to speculate.

"Tell us your speculations," Sherlock said. "We will not betray your confidence. We want to know anything that may be relevant to the case."

Dawlish feigned hesitation. Finally he said, "Draco Malfoy has a lot of clout with Hogwarts' Board of Governors as well as with the Ministry of Magic. For the time being, his son remains a student at Hogwarts, his fate yet to be decided. The Wizengamot was to meet in three days to decide Scorpius' fate. Dexter was to be the prime witness."

"And with Dexter gone…" I muttered.

"The junior Malfoy may be acquitted on lack of evidence."

"Suspicious," I said. "Hermione is aware of this?"

"Oh, most certainly. But as I said, the boy's father is well-connected. To be frank, I believe this is an open and shut case. But the powers that be cannot move openly against the Malfoys, and so they have elected to appoint outsiders —" he gestured to the two of us "— to build an unprejudiced case against them."

Sherlock was, as ever, impassive. He said, "Where were you the night of Dexter's death?"

"In the dungeons. I was — err, brewing a potion for my second year students — a Strengthening Solution suitable for embryonic mandrakes. Professor Malfoy permits me to make use of his Potions classroom. Herbology and Potions are complementary disciplines; many Potions have herbal ingredients."

"Was anyone else there at the time? Malfoy?"

"Afraid not. I was alone for the duration of my visit."

"And what time did you leave?"

He shrugged. "10pm. Give or take. I wasn't paying attention to the time. When I returned to the Greenhouse, that was when I discovered Dexter's body."

A clock chimed, and brooms and watering cans materialized out of thin air, sweeping the room and tending the plants.

"That marks noon," Dawlish said. "I'm terribly regretful, I know this case is urgent and important, but Hufflepuff is playing in today's Quidditch match. As Head of House my presence is required."

Sherlock hesitated. I knew he had more questions swirling in his mind, but there would be more chances to question Dawlish, after we were armed with more knowledge. Sherlock nodded, and we both stood up and prepared to leave. Just as we were at the door, the professor spoke again.

"There's… there's something else you should know. Something which should not concern you, but I want you to hear it from me. Dexter's parents had a bitter and tempestuous relationship. Several years ago their marriage was annulled, and… well, his mother is a beautiful and vivacious witch named Helena Parkinson. A year ago I asked for her hand in marriage. Dexter Zabini is my stepson."