He could hear them from the room that was supposed to be Kate's. Her moans and strangled gasps of "Will" were answered by his groans and heavy breaths that Harry could hear even through the wall.
His parents had stopped sleeping together before he was born, so he'd never had the experience of walking in on them. And St. James was big enough that he hadn't ever had to listen to Dad and Camilla, but he'd seen her leaving in the morning often enough that he was sure they were having sex.
Some people would have been reluctant to meet Kate's eyes the next morning. Harry was brought up better than that.
Three days later, he walked in on them in the kitchen. Wills' hands were under Kate's skirt. Hers were in his trousers. Their mouths were fused together.
It was Kate, leaning back against the counter, who noticed him first. She stiffened. "Will."
Wills kissed her neck. "What?" he said into her skin.
"Will." She was more insistent this time, and accompanied his name with a shove.
"What?" He stepped back and turned his head to follow her gaze.
Their eyes met for a long moment. They were both too well brought up for any further reaction.
Harry turned around and left the flat.
Everyone else was out when he got back, and he started methodically stripping his things from the room he'd been lent.
"What are you doing?"
Harry looked up at Wills leaning in the doorway. "Leaving." He threw another drawer's worth of things into his suitcase.
Wills crossed his arms over his chest. "Is this because of what happened in the kitchen?"
"No," Harry lied. He dropped a book in on top of his clothes. "This was a stupid idea." He yanked the zipper of his suitcase closed. When he looked up, Wills was a great deal closer to him than he had been.
"Where are you going?"
"Home." Harry glanced around the room to reassure himself that he'd packed everything.
"Camilla's moving in."
Harry shrugged. "She practically lives with him anyway." He picked up his bags and looked at Wills expectantly.
"You can always come back," Wills offered, but he looked as if he knew Harry wouldn't. He stepped out of the way and let Harry leave.