I sometimes come to the end of a comic book and say "This handles the first meeting of Winnie-the-Pooh and Paddington Bear in an obvious and predictable way; any fan could have written it themselves." The proper reaction to Doomsday Clock is more like: "This copies the superficial style of Watchmen in an obvious and predictable way; but it is written very much better than any fan would have done."
Which raises the question "Why is Geoff Johns, who can obviously write a bit, wasting his time on this thing?"
This issue establishes a back story for Marionette and Mime, the two Watchmen-universe super-villains introduced in issue #1. This is in itself an obvious and predictable copy of the superficial style of Watchmen; which dedicated several issues to establishing the backstories of individual characters. The flashback to Marionette's childhood copies the style of those flashback episodes in such an obvious and predictable way that it set my teeth on edge.
Doctor Manhattan's dad made watches; and that imagery feeds into the Doc building his weird artifice on Mars; his non-linear perception of time; Einstein's line about becoming a watchmaker; the title of the comic....and so on, to infinity and beyond. Marionette's father made -- I wonder if you can guess -- puppets. (An immigrant-run marionette shop opposite an immigrant-run cut-glass shop seems like something out of the 1930s rather than the 1970s, but possibly all superhero flashbacks take place in that period known as The Olden Days.) When Rorschach was a little boy a group of bigger boys called him whore-son and he stabbed one of them in the eye with a pencil. When Marionette was a little girl some bigger girls called her dad a creepy child molester and the boy from the glass shop over the road smashed one of their heads open with a bottle.
Marionette's father is forced to pass protection money, or possibly drug money, or possibly bribes between the mob and some bent coppers. He does this by, er, hiding wads of cash inside his puppets. He is so ashamed of this that he ends up taking his own life. Marionette finds him hanging in his shop as if he were a puppet himself. And we are all puppets, don't you know, only not all of us can see the strings. I wonder if some day that you'll say that you care?
But once we were told about the origins of Rorschach, Doctor Manhattan or Ozymandias we felt we understood those characters a little more; and the more we understood the characters the more sharply the Watchmen setting came into focus. We saw that Rorschach was not just a vigilante: he believes that good and evil are absolutes but that they were invented by humans and imposed on an amoral universe. This to some extent explains his actions: when he chooses to die rather than compromise his beliefs we understand why. I suppose that this story tells us that Marionette and Mime are very dedicated to each other because of a shared trauma in their childhood; and that they became criminals because Marionette's father was driven to suicide by corrupt cops. But really: we're back in that monochrome universe where "Because a baddy killed his daddy" is a good answer to the question "Why would a brilliant multi-zillionaire dress up as a bat every night?"
Yes, you can blackmail me into empathizing with two characters by telling me that they had horrid childhoods; but the question "Who are these people? Why should I care about them? And how do they fit into the story?" remains entirely unanswered. They are currently hanging out with the Joker's entourage, which merely underlines the fact that they are not very much more than Poundland Harley Quinn knockoffs.
Meanwhile, in the present day, all the villains in the DC Universe or possibly Gotham City are gathered together in a villainous convocation. They get one panel each. Here's the Penguin; here's the Scarecrow; here's one I don't remember.
There are off-hand references to the Green Lantern and all his enemies having left earth and Wonder Woman having been forcibly returned to Paradise Island, which is either a witty allusion to Dark Knight Returns or else isn't.
Then the Comedian turns up and everyone gets shot.