The first thing I noticed was the wet footprint on my kitchen tile.

It was narrow, with a crooked heel and a crescent of dark grit pressed into the grout.

I stood inside the back door holding two bags of groceries, and I knew I had locked that door before leaving for work.

My swimming pool glittered beyond the glass, undisturbed except for a pale blue stain beneath the shallow-end ladder.

A woman was standing beside it with her back to me.

She wore a white sun visor and the bright green sandals I had seen outside the house next door.

I set the groceries down.

“Can I help you?” I called.

My neighbor, Meredith Sloane, turned so quickly that her sunglasses slipped down her nose.

She had been living beside me for four months, long enough to borrow my ladder without returning it and long enough to complain about the sound of my pool pump.

She looked at the towel over her arm, then at the water.

“Your gate was open,” she said.

“No, it wasn’t.”

“Well, somebody should tell you that the water looks dangerous.”

She spoke as though a damaged lock and her presence on my deck were ordinary matters.

I walked around the patio table and saw a small silver container near her foot.

Meredith covered it with the towel.

“What did you put in my pool?” I asked.

“Nothing.”

“Then why are you here?”

“I told you. I thought I saw a problem.”

I was fifty-three years old, a former municipal water laboratory technician, and I had spent most of my adult life identifying problems that other people preferred not to see.

The stain looked wrong for algae, wrong for ordinary pool treatment, and wrong for anything that should have appeared in a clean residential basin overnight.

“Step away from the water,” I said.

Meredith gave a sharp laugh.

“You think you own every inch of this street.”

“I own this yard.”

“That might change.”

The words came out before she could stop them.

I watched her reach for the silver container and drop it into her tote.

“I am asking you to leave,” I said.

“And I’m telling you this pool is a hazard.”

She crossed my patio, pushed the gate open with her shoulder, and hurried home without looking back.

I found a fresh scratch beside the gate latch and a damp mark where someone had braced a foot against the fence.