So this is pretty much a by-the-numbers action flick. But it's got Denzel Washington as the hero, and that's enough to raise it up to four point zero stars right there.
Mr. Washington plays Robert McCall, living a quiet existence in a downscale Boston community (Chelsea/East Boston or somewhere in that area). He takes the Red Line to work at a big-box home improvement store. (Actually, according to IMDB, a defunct Lowe's in Haverhill.) He's affable enough, but essentially a loner.
He's got insomnia, which makes him a regular customer at a seedy all-night diner, where he drinks tea (made from a teabag he brings from home) and reads his dead wife's novels. This leads him to meet "Teri" (Chloë Grace Moretz), a young prostitute who has ambitions to leave her sordid life. (The guy playing Teri's pimp, David Meunier, previously played Johnny Crowder on Justified; pretty much the same sleazy character, except with a Russian accent instead of a Kentucky accent.)
Teri's ambitions are hindered by a brutal beating from Russian mobster/pimp. Which leads McCall to help out, first peaceably, then not so much. It turns out that McCall has (as Liam Neeson would put it) "a very particular set of skills", and he finds himself at war with the entire Russian mob. The Russian mob should have known better.