A/N: For some reason, Rowling states that the Quidditch stadium is to the west of the castle (Harry sees the setting sun reflected in the windows of the castle as he leaves the stadium), and that the Forbidden Forest is also to the west of the castle (at another point in time, the setting sun gilds the treetops as it sinks behind the forest), BUT that the stadium and the forest are on opposite sides. (It's in the Harry Potter Lexicon, therefore it must be true.) I chose to put the stadium in the west and the forest in the east, with the Whomping Willow also in the east.

Chapter 1

Severus Snape lay flat on his back on the east lawns of Hogwarts. The sky lay over him like a smothering dome, with the last scarlet-streaked clouds trailing just behind the Astronomy Tower of the castle. The trees forming the dense border of the Dark Forest were painted crimson too, by the setting sun. It sank heavily behind the Whomping Willow, which quivered, though untouched by any detectable breeze. It would soon be dinnertime, Severus knew, but his lethargy was unconquerable.

And yet why shouldn't he lie perfectly still here on the ground, with the cold grass pricking gently at his back and the lengthening shadow of the Whomping Willow crossing his stomach? He had just finished the second-last exam of his sixth year and he was exhausted.

On top of studying six hours a day, Severus had come close to a complete nervous breakdown this morning- the third time this year, and yet again, Potter and his gang were to blame. They had been up to their tricks again last night; they had stolen of Severus' Transfiguration textbook and hidden it, and he hadn't found it (they had jammed it behind a filing cabinet in Filch's office, Severus had risked his neck getting it back) until right before the exam. They had deprived him of an entire morning of revising for his worst subject, and he had gone into the exam with a rumbling stomach, having missed lunch to break into Filch's office to retrieve his book.

"Brutes and brigands, the lot of them," Severus declared savagely, but he received no reply from the trailing clouds in the sky.

With a sigh he turned his head and peered past the Whomping Willow at the Seville orange of the sun, trying to estimate the time. Then he noticed the murky outlines of two people coming his way, under the shadow of the castle. One was white-suited, neat, and erect, and seemed to be guiding the other figure, who was hunched over and stumbling, as if weak or ill.

Severus got up quickly and ducked into the Dark Forest. Safely concealed by the low branches and scrubby undergrowth, he squinted at the two figures, indistinct in the gloom. They kept coming closer, though there was nothing but the Whomping Willow and the Forbidden Forest on this part of the grounds-were they looking for him? He doubted it; but he shrank back and continued to study the approaching pair as they left the broad shadow of Hogwarts. He had the sun in his face, but now they were near enough for him to recognize Madam Pomfrey, the school nurse, and Remus Lupin, one of Potter's cronies.

Severus frowned. What were these two doing here? Lupin, Severus remembered, had been in the hospital wing since yesterday- he had been given a postponement on his Transfiguration and Astronomy exams because of 'extenuating circumstances.' Severus had assumed it was some life- threatening illness, but if so, what was he doing out here on the east lawns with Madam Pomfrey?

As he watched, the nurse steered Lupin towards the Whomping Willow, which immediately began to flail its huge branches violently. Severus' puzzlement increased. What was this, a mercy killing? The nurse raised her wand and at the same time turned to give Lupin some reassuring words; as a result she missed noticing that the branch she Summoned from the forest thwacked Severus on the back of the head.

He woke two hours later facedown in the dirt. Night had fallen, and the grounds were tranquil and dimly lit by the glowing full moon. Madam Pomfrey and Lupin were nowhere to be seen.

"What's going on?" Severus demanded aloud.

The shrub next to his head refused to answer, although at that moment a terrifying howl rent the air. Severus did not wait to find out from whose throat the scream had been ripped. He picked himself up and ran full pelt, straight past the creaking Whomping Willow, all the way to the front doors, and did not stop until he was safe in the dungeons.

* * * * *

At eleven o'clock that night, he and the other sixth year Slytherins gathered at the top of the Astronomy Tower for their last exam. Severus tried to forget about Lupin. If he could get a perfect mark on the Astronomy exam, he would be the top student in his year. If he didn't take the top place, he would almost certainly be flogged by his father. Severus wished he had spent more time revising Astronomy. He got his blank star chart and set up his telescope to point at the sector he was assigned.

"Miss dinner to hide in the library, Snivellus?" came a whisper from his left. Severus looked up sharply at James Potter, his archnemesis and Gryffindor boy wonder. "Food less important than marks?"

"Yes, Potter, I was studying. You should try it some time. Maybe good marks would lend you some integrity."

There was a snigger from Potter's other side, and Sirius Black, Potter's best friend, leaned over and hissed, "Listen James, Snivellus the Slytherin is telling us about integrity!"

"What a joke," piped up Peter Pettigrew from behind Black.

"Would you all shut up?" hissed the person just beyond Pettigrew.

They all leaned forward to peer at Lily Evans, another Gryffindor. Severus had disliked her since the moment she'd defiantly told him she was Muggle- born. "This is a private conversation, Evans," he said coldly.

"Some of us are trying to write an exam," Evans said, equally frigidly. "Shut up or I'll tell Professor Sinistra you're cheating." She turned back to her telescope.

Pettigrew looked worried, but Potter whispered confidently to him, "She wouldn't tell, even if she is a prefect."

"And a Mudblood," Severus murmured to his star chart, not intending anyone to hear him, but Potter's face darkened and he deliberately jogged Severus' telescope with his elbow.

"Hey!" Severus had spent a long time painstakingly adjusting the right ascension.

"Oops," smirked Potter.

"Pettigrew! Potter!" barked Professor Sinistra, the young but strict Astronomy teacher. "Have you been assigned sectors yet?"

"No, Professor."

"Then get over here at the double- and stop distracting the other students! This is an exam!"

Potter got up, but he didn't leave right away. Instead he bent over and hissed in Severus' ear, "Never insult Evans. You're not good enough to polish her shoes."

"Does she have any? I would she think she wouldn't have the money," Severus responded coolly without turning his head.

Potter sucked in his breath and would probably have done something drastic to Severus' face, had Evans not intervened, whispering angrily, "Just forget about it, Potter, I can stand up for myself!" Leaning round Potter she snarled at Severus, "Shut up! I'd rather have friends than expensive footwear!"

She spun back round without waiting for an answer. Potter swaggered away, Pettigrew following close at his heels.

Fuming, Severus righted his telescope and began to fill in the Pleiades. Out of the corner of his mouth he whispered to Black, "So where's the fourth stooge?"

"I beg your pardon?" Black was trying to sound scornful, but the exaggerated formality told Severus he was rattled.

"Lupin," hissed Severus.

Black's hand slipped, causing his quill to skate across his chart and effectively obliterate the constellation Orion. "He's ill. Why do you care?"

"If he's ill, why isn't he in the hospital wing?"

Black straightened up and stared defiantly at Severus. "He is."

"No he's not," Severus replied calmly, measuring the parsecs between the stars Alcyone and Asterope. "I saw him roaming the grounds this evening with Madam Pomfrey. Come on Black, what mischief are you idiots up to this time? Did you Polyjuice Madam Pomfrey or something? Are you playing that stupid game with the Whomping Willow again?"

Black started and knocked over his telescope. "Damn!" They both glanced towards Professor Sinistra, but fortunately she was still busy with Potter and Pettigrew and hadn't heard the commotion or the curse. Evans glanced their way disapprovingly, but didn't seem to have heard been eavesdropping. Black hurriedly set his telescope straight again and aimed it more or less at the right star cluster. Then he smiled at Severus. "Want to find out?"

Severus frowned, instantly suspicious. "What are you talking about?"

"There's a tiny knot near the base of the trunk," Black said. "Reparo!" The shards of the shattered lens of his telescope sprang back into place. He grinned, vaguely menacing. "All you have to do is touch that knot with a long branch. The willow will stop whomping and you'll see exactly what we've been up to."

A distant howl reached their ears, but Severus was thinking too hard to pay it any attention. Was Black being serious? Could this be a trick? The branch bit was probably real: he supposed it had been one of those that had knocked him unconscious earlier. He was skeptical of Black's motives in revealing the secret to him, but he very dearly wanted to find out what this was all about. Black raised his eyebrows. "Or are you a coward, Snivellus?"

The reference to a possible danger should, of course, have alerted Severus to the imprudence of the excursion, but in this instance his curiosity prevailed over his common sense. "I'll do it."

"Tonight," said Black.

"Fine," Severus said shortly.

"Snape! Black!" They both jumped at the sound of Professor Sinistra's voice. "I trust you are both aware that talking during exams is forbidden?"

"Yes, Professor."

"Then be quiet and sketch," Professor Sinistra said sternly.

Evans smiled smugly. Potter and Pettigrew returned to their places, looking curious, but Severus had already turned back to his star chart. Even as he carefully labelled his constellations, he was planning how to sneak out to the Whomping Willow.

At midnight the last exam of the year was over. The sixth years hastened from the Astronomy Tower, cheering- quietly, so as not to wake the slumbering inhabitants of the castle. Black sidled up to Severus as they hurried down the staircase to the seventh floor. "Not having doubts, are you?"

"Of course not," Severus scoffed. James Potter caught up to them then, frowning.

"Chatting together in exams, in the corridors... What, are you two dating now?"

Black fairly shrieked with laughter. Severus wrinkled his nose. "Gits," he muttered, striding ahead.

As he left the Gryffindors he heard Potter ask, "What were you talking about, anyway?"

"Just wait and see," Severus heard Black reply gleefully, before he turned the corner and set off for the front doors.