"Rats," by Tom Savage, in Alfred Hitchcock's Mystery Mystery, September/October 2018.
Are you familiar with the term logline? Think of the one sentence description of a movie or a TV show you see in TV Guide or Netflix.
Here's a logline for a short story:
A senior citizen combats the bad element that is taking over the neighborhood.
I have probably read a dozen stories that fit that line. Of course, there are no new plots, just new things to do with the old ones. Is the senior alone or does he have allies? What kind of plot does he dream up? Does he succeed or fail? I remember decades ago reading a story in which an older woman, tired of having her purse snatched, carried a hand grenade in the purse with a string tied from her wrist to the pin. A mugger grabbed the purse and three seconds later, BOOM.
But that's not Savage's idea. Alice lives in New York City. She still teaches a few days a week at a middle school. She lives in a co-op which has always been neighborly and well-maintained, but recently a dozen apartments were purchased by a Russian mobster. Worse, he has moved his nephew, "a huge, unkempt, unfriendly, leather-jacketed hell-raiser named Georgi," into one of the apartments. Things start to go downhill. Alice's friend Marco, a retired circus performer gets robbed and beaten, and that's not the worst of it.
But when Alice sees the janitor putting out rat poison she gets an idea on how to solve the Georgi problem. If only she can get Marco to go along with it.
I did not see the ending of this one coming.