The title is in italics because it's the name of a TV show: yet another superior BBC crime drama. The twist, or gimmick you might say, in this one, is that the detective--the "River" of the title--has some pretty serious mental problems: he sees and has conversations with people who aren't really there. The story revolves around his efforts to solve the murder of his partner (working, not romantic). As is typically the case with these British efforts, the writing, acting, and general production are really fine, with a very convincing cast of characters. River is played by a Scandinavian actor, Stellan Skarsgård (I think that funny "a" is Swedish, right?), who apparently has a good reputation, although this was the first I'd heard of him. Also apparently in possession of a prior and favorable reputation is the writer/director, Abi Morgan, who wrote the Margaret Thatcher biographical picture, The Iron Lady, which I haven't seen (though I would expect any portrayal of Thatcher to be negative and not necessarily fair).
It occurs to me as I write this that there are only one or two characters who didn't engage my sympathy to at least some degree, no matter how flawed they are--and one of those is a 19th century serial killer who is a very unwelcome visitor to River. As crime stories go it's not particularly lurid, and although it can fairly be described as "dark" it is not excessively so. I found the ending very powerful, as did my wife, and we aren't necessarily affected by the same things. We started watching it only because we were looking for something that wasn't more than an hour long and it appeared in a list of recommendations on Netflix based on other things we'd watched. We had no idea what to expect and were soon very much engrossed.
It may only be available on Netflix in the U.S. It's tagged as a "Netflix original", which I guess is not exactly right.