Synopsis: Barry Allen is nothing if not philanthropic. Helping with a local charity fundraiser, he, as The Flash, vibrates himself at amazing speeds so that he appears to perform "magic" and disappear. When he stops his "trick", though, he finds himself in a different place...altogether.
As he investigates the city he's now in, he finds it's called "Keystone City"--the very same city that was the home of the fictional Jay Garrick he used to read about in comic books.
Taking a chance that his years spent reading All-Star Comics might not have been a total waste of time, he looks up "Garrick, Jay" in the phone book...and discovers he's a real person in this city. When he goes to visit, he notices immediately that it really is the Jay Garrick of the comic books, so he immediately reveals his identity and what he knows of Garrick's origin. Together, they work out that they are from two different Earths, which have very similar, yet distinct, evolutions.
Impressed, the now-retired Garrick decides to enlist Allen's help in solve several crimes plaguing Keystone City. The two Flashes strike out in the direction of The Fiddler, The Shade and The Thinker. They divide the workload and tackle different the different criminals individually.
First tackling The Shade and The Thinker, neither Flash is successful. After their individual defeats, they join forces against The Fiddler. He, though, appears to defeat both of them with one of his musical spells, forcing them to rob jewels for him. When the other two villains arrive on scene to warn The Fiddler that there are now two Flashes, Garrick and Allen suddenly turn on all three villains and overwhelm them. It turns out they've put the jewels they "robbed" in their ears, thus blocking The Fiddler's spell.
Having helped Jay successfully come out of retirement, Barry attempts to go back home by reversing what he did to get to Keystone City. When he's successful, both he and the reader can only imagine what might happen in Jay Garrick's future, and what the rest of this mysterious "alternate" Earth might look like.
Interestingly, Allen works out that Gardner Fox, author of this story and many issues of All-Star and Flash Comics is a kind of bridge between the two Earths. His dreams on Earth 1 are actually channelling real events on Earth 2. Thus, Gardner Fox himself becomes a part of the DCU through this story.
The final issue of Flash Comics is revealed to be the last adventure the Flash (I) had, prior to this one. Now, sure, some 12 years have passed in Garrick's life since then, so there's some kinda story to be told, but his life as the Flash has been effectively "on hold" the whole time. One of the interesting such tidbits that has happened in the intervening years is that he has married Joan Williams, which is revealed for the first time in this issue.
Synopsis: Earth 2. A comet hits the sun. A meteorite hits the Arizona desert. The sun begins to send out deadly radiation. The meteorite absorbs it. Only...the meteorite disappears, and Jay Garrick can't find it.
Believing that it's likely that the same event occurred on Earth 1, Garrick vibrates to the other side and crashes a costume ball that is being attended by Barry Allen and his girlfriend, Iris.
Barry's problems this night are much smaller--only he doesn't know it yet. While enjoying the charms of the good Miss West, Barry is unaware that the charity has been robbed by Captain Cold and The Trickster, who are now making their getaway. Luckily, Garrick sees them escaping and begins to do battle against them, instinctively. The furor distracts Barry from Iris' attentions, and he's soon joining his counterpart as The Flash. They aren't able to capture the villains, but they do run them off.
The immediate problem temporarily insoluble, Jay tells Barry why he's really come to Earth 1. They hatch a plan to take care of both their problems.
While the comet has not yet struck Earth 1's sun, the meteorite has landed in Arizona. They dig it up, and then cut it into smaller, diamond-size pieces. Publicizing the presence of "a bag of diamonds" under minimal guard, they lure Captain Cold and The Trickster into leading them back to their lair. There, the two Flashes, now armed with surprise and a battle plan, easily defeat the villains, regaining both the charity's money and the priceless meteorite fragments.
Jay returns to Earth 2, saving that world from sure destruction. The meteorite is then synthesized for use on Earth 1, should a similar comet ever strike Earth 1's sun.
Synopsis: While reading a copy of All-Star Comics #37, Barry Allen notices strange lights appearing overhead. Speeding around North America in response, he discovers a pattern: lights have begun appearing over the home cities of key JSA members.
Sensing this must mean something is wrong on Earth 2, he uses his super speed to "cross over" to talk with Jay Garrick. A light matching what he saw on Earth 1 then appears over Keystone City. Racing back home, Jay is immediately imprisoned in a stasis cube. Still free, Barry goes to the source of the light and disables it, freeing Jay.
The pair discover that the person controlling these lights is none other than Vandal Savage, the villain who made his first appearance in the very same All-Star #37 that Barry was reading back on Earth 1.
They go to his lair, only to be rewarded by a blast from a device that makes the Flashes fight each other. Jay gets knocked out, and Barry is led away from the scene of the fight--into a trap. Now able to focus his mind again on the original mission, Barry eludes capture and frees Jay and the various JSA members trapped by Savage. Together, they overcome Savage and take him to jail.
The JSA members, together for the first time in a number of years, then agree to come out of retirement and reform the original "super team".
Prior to this issue, there was only the implication that other heroes besides Jay Garrick lived on Earth 2. This issue positively demonstrated, for the first time, that any character that had previously existed in the 1940s probably still existed on Earth 2. Except for Jay Garrick, who had been seen a couple of years earlier, this was the first appearance, since the dawn of the multiverse, of the key members of the JSA.
Interestingly, it's also the origin story for Vandal Savage.
This issue reinforces the idea, now twice seen before, that comics published during the 1940s were just as available to residents of Earth 1 as they were to us, the readers. But, from the perspective of Earth 1 characters, they're also shown to be a "real" history of events on Earth 2, rather than pure entertainment.