Rebirth begins with a mystery villain attacking the Central City Police lab where Barry worked and experienced the accident that gave him super-speed. The mystery villain, after he uses a sword with the Flash logo as a weapon to dispatch several police scientists, has a cosmic kind of knowledge that leads him to show up just before lightning hits the same shelf of chemicals this apparently gives him super-speed. He also claims to be the one who decided to bring Barry back from the dead. When we see the villain's face, it is a ghastly skeletal grimace, and so if it's anyone we know from the past, the identity remains hidden for now.
There is, therefore, a plot unfolding in the present, in which the concepts of Death and Super-Speed seem to be intertwined as though they were two sides of the same coin. The mystery villain has some sort of executive role in an unknown plot. Of this, we know the following:
a) The residents of Gorilla City are participating in a ritual that seems to anticipate events in the Flash-related plot.
b) The speed cult of Savitar (a speedster villain from the Wally West era) and the Lady Flash (Christina Alexandrova) are reawoken by what's going on. They separately attack Barry Allen and separately die, to his dismay, at his touch.
c) When Barry's touch causes the villain speedsters' deaths, very speedster with a connection to the Speed Force experiences a shock of some kind which gives them seizures, as though an electrical surge had risen up through the Speed Force.
d) The Black Flash (who, we are told, is synonymous with the Kirbyesque Black Racer who claimed Darkseid in Final Crisis) skids to a halt -- his own death -- in Fallville, Iowa, which is Barry Allen's birthplace.
e) Perhaps because of Lady Flash's death, perhaps because of Barry's arrival at the corpse of the Black Flash, Barry transforms at the end of Rebirth #2, becoming the new Black Flash.
Maybe this mystery has a lot more to it, and maybe it doesn't. It could wrap up simply (and soon) by revealing a villain who has magical/cosmic/future insights and found a connection between Speed and Death, cursing Barry with his role as a new Grim Reaper. The exact identity of the villain is still up in the air, but the facts we have so far seem to sketch out the plot as well as the implications. I would say there's maybe half a mystery left. Even though the themes and the characters seem to overlap partially with Darkest Night, chances are the stories will disentangle themselves. Why write one story when you can write two? Perhaps the plots will relate, obliquely, but they will not be one and the same.
Meanwhile, writer Geoff Johns is also reinventing Barry Allen's past to some degree. And this is at least partly necessary. When Jay Garrick recounts meeting Barry, we must have an account that matches the one-world universe. Instead of Barry Allen traveling to Earth Two to meet Jay, he presumably just ran from Central City to Keystone City.
But Johns is also reinventing Barry Allen's background. When Barry compares his return to Hal's, he reflects that Hal's death took place in ignominy, whereas when Barry died "Everything was fine." Well, that's not how it went down in 1985. Barry's last solo issue -- twenty-four years ago -- opened with the police announcing that he had escaped from his jail cell. In the eyes of the law, he was, like Hal before his swan song, a murderer. Also, Johns is possibly removing the facial transformation that Barry underwent before his disappearance -- not just restoring his face post-resurrection, but removing even the history of it having happened.
Additionally, Johns has created a new vaguely Batmanlike childhood for Barry in which Barry's father died in jail for the murder of Barry's mother -- a very different history than the original, in which the Allens were both present and alive during Barry's final solo issue. Henry Allen had been possessed by the spirit of the deceased Flash foe The Top, but was fully recovered. During the second Flash series, it was stated that the elder Allens both died after Barry's departure. Now we have a very different reality, and some of the old stories no longer match. So ultimately, this series is not only a rebirth in the sense of bringing Barry back from death, but it retelling his story's beginning.
Given the new approaches Johns is taking, readers who feel like they need to scan dozens of old back issues can sit back and relax. There is a wealth of old material there, but the road ahead is cutting its own path. If this is true to the success that Johns has had with Hal Jordan, we will see a new take on the past, planting new facts into the old origin that conveniently come due for repercussions in the stories to come.