This is pure old fashioned sci-fi pulp. It's a lot of fun, but very silly.
Plot Summary: A man wakes up to find he's been turned into a robot (although bearing in mind this was written in 30's, the word Robot isn't used). After finding a friend, in the same predicament, they take a tour through the city to find that most people weren't so lucky: The city is full of broken metal hunks, apparently people who didn't make the transition.
So the first 1/4 or so is pure post-apocalypse survival, and it's quite fun. Then they start to figure out who and what is behind their transformation, and it turns into a "fight for the survival of humanity, or something like humanity" battle against a superior intelligence with superior numbers.
The plot is convoluted and while it's very gung ho, it's got some nice twists: The typical American hoorah saving the world a la Indepence Day, is in this case backed up by a hilarious appearance of the Australian Navy (who weren't as it happens, turned into robots, although they were turned unilaterally blue!)
Also nice is that there are two major female characters, and although there's a bit of typical mid-last-century "little women" crap ("she tossed the ball in that underarm motion that is all women can do in approximation of a throw") they are smart and tough and sassy, and not just there to provide someone for the boys to keep saving.
Warning for Grim however, because I am sure you spotted the word Robot in here, you will find the ending tragic:(show spoiler)