Random writing nr 2

When I come home from school I see an envelope at the ground and it was named to me. I opened the letter and in the letter it stay:
"Midnight, on the bridge. Come alone."

Who had write it? Why should I was at the bridge at midnight, alone? I didn't understand. I don't want to be there at midnight, but what is happening if I'm not there? So half past eleven I had decided me to walk to the bridge.