It's Ennis's take on James Bond. His name's Jimmy Regent, i.e. next door on Monopoly. He's suave, he shags all the girls and he's unstoppable... in Book 1. In Book 2, he's had a shock and he's not himself any more.
He'll still fight baddies, though. He also has a sidekick, Nancy McEwan, who's unimpressed with him and is arguably the book's real action hero. Let's face it, there's little doubt about what will happen when someone fights 007. Nancy, though, gets the shit kicked out of her and then gives ten times as much back.
You've already guessed the plot. Look at the title. Imagine the fun you could have writing James Bond and that idea. Someone else must have done it before, surely, but that doesn't mean Ennis's version isn't lots of fun.
Interestingly, it's not just a 007 story with the serial numbers filed off. It's neither regurgitation nor parody. I think Ennis likes James Bond and he's not bashing the character, but the story takes its own directions and creates its own conclusion. Jimmy Regent stops being just another iteration of his source material, although in fairness he's not a clone even in Book 1. He swears a bit more (at least than the movies) and he's a bit more chatty and opinionated.
This series answered a question for me. I'd been wondering what would happen when Ennis met political correctness. I'd been reading Garth Ennis comics from the past thirty years and they'd been full of gross ideas and gratuitously offensive swearing that could these days be called something-phobic. However, I hadn't yet read anything of his from the age of micro-aggression, safe spaces, gender fluidity, checking my privilege, mansplaining, etc. Well, my wait is over. He's deliberately put all that into this book, with sardonic observations and jokes from his equivalent of the famously dinosaur-like 007.
This is a good story and far more interesting than just another Bond parody/pastiche. (Those are ridiculously easy to write, but there are a million of the bloody things out there and you can never tell in advance if they're going to be funny or tiresome.) It's light and funny, but not an Ennis pisstake. The drama has a reasonable amount of weight. It also avoids the non-ending that a Bond fanboy would expect. (I like Bond films, but until Daniel Craig they tend to feel like interchangeable instalments of a formula.) The art's also good, with Braun's lively, expressive faces being a great fit for Ennis's storytelling.
Would I recommend this? Sure. Should everyone rush out and buy it today? Of course not, but you could do a lot worse.