There was a smell of wet wool and nervous sweat in the lecture hall. Three hundred students sat in silence on the steps, watching the man on stage walk back and forth like a metronome.

Dr. Elias Thorne stopped in the middle of the spotlight. He was dressed perfectly in charcoal wool, and his silver-framed glasses hid his eyes by reflecting the lights above.

"Raise your hand if you remember exactly what the person next to you is wearing," Elias said in a smooth voice that carried without a microphone.

There was a pause in the room. Some hands went up.

"Stop looking," Elias said in a sharp voice. "Just keep that in mind." He pointed at a young man in the first row. "You. The girl on your left. What colour is her scarf?

The boy stammered. "Uh, red? Possibly maroon.

"It's blue," the girl said quietly, looking down at the silk around her neck.

Elias smiled. It wasn't a warm smile; it was the look of a mechanic who had just found a loose bolt. "Certainty," he said to the room, "is a flaw in biology." It is a way to stay alive. You don't know the scarf is red; you just think it should be because your brain is filling in the gaps in your peripheral vision with what it thinks it should be. You are all telling stories about your own lives that aren't true.

He looked at his watch, a vintage Patek Philippe that was perfectly in sync with the second. Five o'clock.

"Class is over." Don't lie to yourself too much this weekend.

There wasn't much applause as the students left. Elias didn't care. He didn't care if people liked him; he just wanted to be right. He grabbed his leather bag and went out the side door, into the cold wind of October in Chicago.

His routine was like a fortress. There was a black sedan waiting. It took exactly forty-two minutes to drive through traffic to the top of the Willis Tower. It took sixty seconds to get to the 90th floor by lift.

He went into his flat. It was a minimalist temple with white marble, black leather, and windows that went from floor to ceiling and looked out over the city's grid. No mess. No disorder.

Elias threw his keys into the ceramic bowl that was by the door. Clink. He put his coat on a hook. Swish. He went to the kitchen island to get a glass of sparkling water. Fizzy.

He stopped moving.

There was a coaster on the island. A plain cork coaster.

Elias didn't use coasters. His countertops were made of quartz, which doesn't stain. He was by himself. The cleaning service came on Tuesdays, and today is Friday.

He walked up to the thing carefully. There was a glass of water on top of the coaster. The ice inside hadn't melted yet. There were drops of condensation on the edge, and one drop followed a path down the side.

Someone had made this drink. Not long ago.

Elias looked around the room. There was no place to hide in the open-concept living room. The corners were dark, sharp, and empty.

"Hey?" he yelled. His voice didn't sound as authoritative as it did in the lecture hall. It sounded small next to the glass walls.

Be quiet.

He reached into his jacket pocket and felt the cold steel of the tactical pen he always carried with him. This was a paranoid habit he had from making enemies in court. He walked towards the bedroom, looking in the bathroom, the closet, and the space behind the curtains.

Nothing. The flat was locked up. The electronic lock on the front door didn't show any entry logs on his phone app except for his own arrival just now.

He went back to the kitchen. He looked at the glass of water. Maybe he poured it before he left this morning? No. The ice. The ice was new.

Elias touched the glass. It was really cold. He raised it.

There was a small, folded square of paper under the glass on the cork coaster.

His heart raced against his ribs in a frantic way. This couldn't be done. This broke the laws of physics and safety. He opened the paper.

There was a receipt. A box of 50 9mm bullets. Tomorrow is the date. At 9:30 AM. Buyer: Elias Thorne.

Elias let go of the paper. He stepped back, his breath catching. He didn't have a gun. He didn't like guns.

His phone buzzed in his pocket, making a loud noise against his hip. He jumped up and tried to get it back.

Number not known.

He looked at the screen for a long time before moving his thumb to answer. He didn't say anything.

"Dr. Thorne," a voice said. It was digital and synthesised, so it didn't have any gender or accent. "You forgot your drink. It's getting hot.

Elias looked at the glass that was on the counter. The condensation was forming pools.

"Who is this?" Elias yelled, trying to calm down. "How did you get in here?"

The voice said, "Wrong question, Professor." "The question isn't how we got in. The question is... why do you think you left?

The queue was cut off.

Elias looked out the window. The skyline of Chicago was there, shining and huge. But as he watched, the lights on the nearby skyscraper flickered. Not a blackout. They blinked in a pattern.

Short. Long. Short. Short.

L.

Long. Long. Long.

O.

Short. Long. Short.

R.

Long. Short. Short.

D.

LORD? No. He was reading it wrong. He blinked and rubbed his eyes. The building was dark when he looked back.

He went back to the counter. There was no more water in the glass. The coaster was missing. The receipt was missing.

There was only one fresh, muddy boot print left on the white quartz counter.

Elias looked at his own feet. He had on dress shoes that were clean and shiny.

He wasn't the one who had walked through the mud.