Half an hour later, when tempers had cooled sufficiently enough for Snape, Sirius and Lupin to disappear off into a corner to discuss what would happen next, Harry sat down on the table and thought hard. He wasn't entirely sure what he was trying to think about, but he knew there was something in there somewhere, eluding him, and it was something to do with Ron.

Since fighting the Selkie, Ron had been acting remarkably strangely – for Ron. He had told no one about what had happened under the little stream with the monster; and, while Harry was tempted to put this unusual display of modesty down to Snape's potion, there was something else nagging away at his mind like a dog with a bone. Suddenly it clicked.

Ron was sitting down at the table. His elbows were splayed apart and his head was resting slumped in his hands, and he didn't appear to have realised that he was blue yet: he was gazing off into the distance, ignoring Harry and the others. Quietly, Harry slid down from his perch and crept over to Sirius.

"Did – did you give me the same potion as you gave Ron?" he asked, in a whisper. "When the Selkie bit me?" He noticed Sirius exchange a quick look with Lupin, who raised his eyebrows, before replying.

"Yes – we were just discussing that…"

"Then why haven't I turned blue?"

"It's his complexion," spat out Snape. He appeared to think that his potion-making ability was being called into question. "His ginger hair. And he had a far more concentrated dose than Potter. I told you: this is just the after-effect – it will wear off soon."

"Have you ever known it to have this effect before?"

"I've never given it to anyone before! How should I know? Why? Are you implying that my – "

Lupin shushed him with an impatient hand. "I'm not implying anything, Severus," he said. "I'm just trying to work out what's wrong."

They were, conveniently, stopped from further conversation by a small shriek from Hermione.

"Whatever's the matter?" exclaimed Sirius, turning round.

Hermione was pointing a shaking finger at Ron. Because he was clutching at his face with his hands, devoid now of gloves, his eyes widening in panic.

And from his mouth were growing long, yellow, fangs.

"Another after-effect of your potion, Severus?" enquired Lupin, with a calm that made Harry blink. Here they were, in the middle of a deserted hut in the middle of an enchanted forest, with his best friend blue and developing serious dental problems. Now was possibly not, he thought wildly, the best time to be calm.

"Isn't anyone going to do anything?" he shouted. "Somebody! Help Ron!"

"Well – I would, Harry," said Lupin in the same calm voice, "Only that isn't Ron. I hadn't realised until a minute ago, but – well – I'm afraid we have a problem."

"I know we do!" squawked Harry, trying to pull Ron's hands away from his mouth and giving up as his blue head turned green and horse-shaped.

"…Selkie…" Hermione whimpered, backing into Sirius.

"Well actually no," replied Lupin, as though he were taking a class of Defence Against Dark Arts at Hogwarts, talking Hermione through identifying a certain creature. "Not a Selkie. You see – this is, in actual fact, a Brollachan – a rare form of the Boggart. As you know, the Boggart takes on the form of whatever its victim is most scared of: the Brollachan adopts the shape and traits of its last… er… victim."

"… Victim…" echoed Hermione.

Harry suddenly felt very faint round his knees.

"You mean – you mean that isn't Ron?" he asked in disbelief. "You mean – Ron's dead?"

Lupin shook his head. "No," he said. "The Selkie was the last thing the Brollachan killed, and so he took the form and characteristics of that. He must have damaged Ron, I suppose, which is why he looked like him for a while – but he's turning back into the Selkie, Harry, which must mean Ron is still alive. I wondered why he was blue. I – er – I think maybe we ought to sort this out now," he added, as the Brollachan made a dash for Snape. It stopped just short of the professor, however, and stared at him instead. Harry didn't envy Snape: the goat-like eyes were spinning horribly. There was a stifled pause, as no one dared even breathe, and the Potions master struggled to keep eye contact with it for a while.

Then, in a sudden movement that shocked them all, Snape had leapt behind Lupin with a surprising degree of athleticism, grabbing the startled man's arms and swinging him round in front of him like a shield. He gave him a push towards the Brollachan.

"You're the Defence Against Dark Arts expert," he hissed. "Nice – nice Brollachan." This was directed to the monster, who was still hungrily eyeing the Potions master.

"Er – fine," said Lupin. He was trying to extract himself away from Snape's painful-looking grip, without much success. "Do you think you could let go of me now, please, Severus?"

Suddenly realising who it was he was holding on to so tightly, Snape snatched his hands away as though they burnt him, and jumped back. If the situation hadn't been so terrible, Harry would have been tempted to laugh. But, as Lupin turned to face the Brollachan, wand in his hand, Harry did no such thing. Nothing on earth could at that moment have induced him even to smile: he was trying hard to come to terms with the other problem they now faced.

Because if that was a Brollachan in front of him trying to bite Lupin, where was Ron?