It was an ordinary Saturday afternoon. Father was away on a business trip, and Mother had left the house for a gathering. There was still some time before Han Yu-seul was due back from school.

Guest entered Han Yu-seul's room for no particular reason. An unlocked drawer. That drawer, which I usually wouldn't have given a second thought, caught my eye strangely that day. Half out of curiosity and half out of mindlessness, my hand moved, and inside was a neatly organized notebook.

The first few pages seemed like nothing special. Guest's school commute times, frequently visited convenience stores, favorite snacks, a list of close friends. It was at a level that could be passed off as notes a family member might keep to look after their sibling.

But the more I turned the pages, the stranger it became.

'April 12th. Went straight to the study room after school. Came out at 9 PM.'

'Route for returning home alone: Main gate → Crosswalk → Alleyway → Apartment back gate. Uses this path every week.'

It was a record. A systematic and persistent observation log, closer to a surveillance report. And the short sentences interspersed throughout made Guest's fingertips freeze.

'Must avoid times when there are no witnesses. CCTV blind spots have already been identified.'

It felt like my heart was dropping to the floor. The handwriting was neat and orderly, and the sentences listed facts without emotion. It was the handwriting of someone checking off an execution plan, as if verifying a checklist.

The final page.

'Not much longer now.'

It was just one short line, but its weight pressed down on Guest's chest like a lump of lead. This was a murder plan. One that detailed how to kill me without leaving any evidence.

My hands shook. I knew I had to put the notebook back, but my fingers wouldn't obey.

Just then, the electronic beep of the front door lock sounded.