“No. Right. Yeah. Of course,” Heather said. She laughed, but it was a high-pitched chirp of a thing that was so far removed from her normal laugh Logan initially thought she might have stubbed a toe. “Let’s keep it professional this time.”

“Works for me,” Logan said.

Heather tucked her hands into the pockets of her trousers, her thumbs hooked over her belt. “Right, well. Billy, then. You don’t think he did it, do you?”

Logan shook his head. “No. I mean, I knew he was tied up in it. I just didn’t know how.”

“And you believe his story?” Heather asked, the tone of her voice making it clear that she didn’t. “You actually believe what he said.”

“I think so, aye,” Logan said. “I believe it more than I believe him tying up four people and murdering them all.”

“He was a Biology teacher,” Heather pointed out.

Logan took a bite of his Twix and ruminated on the point while he chewed. “And? Are Biology teachers more likely to be murderers? Did I miss that memo somewhere?”

“I had a look at the pathology report. The placement of the wounds. Someone knew where the heart was.”

“Oh. That.” Logan moved his tongue around inside his mouth, scraping bits of toffee from his teeth. “There’s a big difference between knowing where the heart is and being able to plunge a dirty great knife into it.”

“I still think we should be treating him as a suspect.”

“Aye, well, it’s no’ your case,” Logan said. “He’ll tell Uniform where his wife is, if he hasn’t already. CID can handle the details. Billy’s no’ our killer.”

“So who is it?” Heather asked.

Logan scrunched up the chocolate bar wrapper. “Don’t know yet,” he admitted. “But I’m going to talk to Frankie Cowan again in the morning. Put a bit of pressure on. Use the rape angle.”

“I’ll come with you. You’ll need backup,” Heather said. “He might look like a wee scrote, but Frankie’s not someone you want to mess with.”

Logan nodded. “I heard that, right enough,” he said. “But I’m sure I’ll cope.” He pointed to his coat. “Watch that for me, will you? I’m dying for a pish.”

He headed off without waiting for an answer, throwing open the door and striding away through corridors so familiar his feet could navigate them all by themselves.

Heather sat on the edge of the table and watched him go. Before today, he hadn’t set foot in the building for a good two years, yet he moved like he owned the place. More than that, even. It was like the building—the whole station—was an extension of himself, and if he raised his voice the walls would tremble and shake.