Harry sat on the bench of the Gryffindor House table, gazing intently at his copy of Advanced Potion Making. He was examining the Sectumsempra scrawl at the top of the page. Suddenly, and with no warning, Hermione spoke up.
"Harry," she whispered, not gaining Harry's attention. She spoke up slightly when she noticed. "That's Katie." Harry looked up from his book and at Hermione instead. "Katie Bell!" Harry stood up from his seat and snapped Advanced Potion Making shut. He followed Katie to her friends and started to speak.
"Katie, how are you?" he asked, and Katie looked skeptical.
"I know you're going to ask, Harry...but I don't know who cursed me," she said remotely. At this very moment, Draco Malfoy walked in and shoved his way through the crowd. He saw Harry chatting with Katie and stopped dead in his tracks. "I've been trying to remember, honestly, but...I just can't."
Harry stopped his conversation at once and looked back at Malfoy, who had turned an eerie shade of white that matched his hair. He took a step backward and then rushed off somewhere. This was Harry's chance. While Katie was distracted talking to her friends, Harry followed Malfoy in quick haste, his book still under his arm.
Harry followed Malfoy the entire way to where he was going, only to see him stop in front of a men's restroom on the seventh-floor corridor. He followed him and stood back in the entranceway. Malfoy flung himself close to a sink and pulled off his overcoat, then gazed at himself in the mirror and splashed some water on his face. Harry stepped a bit closer and pulled out his wand, tucking his book in his robe pocket. "I know what you did, Malfoy." Malfoy whipped around on the spot, surprised to see his rival by House standing just feet behind him. "You hexed her, didn't you?"
There was an uneasy silence. Then-
Malfoy flung his wand forward at Harry, his hex missing him by inches; Harry pointed his wand at Malfoy and thought Levicorpus! in his head, but Malfoy avoided the spell and raised his wand for another. After a bit, water was spraying everywhere, and neither wizard could see a thing. To ensure the duel had ended, Harry looked beneath the restroom cubicles, and Malfoy shot a surprise heat Harry that he narrowly avoided. Harry stood up and dashed in Malfoy's direction, pointing his wand straight at him.
"Sectumsempra!" Harry shouted, and a glowing white orb shot Malfoy square in his chest, knocking him into the water with a great splash. The Boy Who Lived walked forward to see what was wrong, as he could hear Malfoy choking and spluttering in the distance. He approached, and what Harrysaw shook him to his core.
Malfoy was lying spread-eagled on the waterlogged floor, blood covering his robes and puddling around him. Harry stepped closer and saw, up close, that Malfoy's entire body was shaking violently, as though he were recovering from a bout of the flu. Harry felt a whoosh of cold air brush past him: Snape had dashed past the Gryffindor and was now looking at Malfoy, then at Harry, his black eyes narrowed. Harry, not knowing what trouble he had gotten into, used this moment to escape to the Gryffindor common room.
In the meantime, Snape knelt down beside Malfoy and traced his wand over the Sectumsempra wounds, muttering "Vulnera Sanentur" with each trace. The flow of blood eased significantly, the wounds knitted, and the excess blood was siphoned off of Malfoy as an individual.
A/N: This scene was taken from the movie, not the book. Just thought I'd clear that up.