Plot: Babylon 4, the predecessor to B5 which had disappeared, reappears, having travelled forward through time four years. B5 sends shuttles to rescue the people there, as B4 is still going through a mysterious anomaly in which people see brief bursts of the future or past; they are "unstuck in time".
On B4, an alien named Zathras says that he works for The One and is trying to take the station through time to use as a base of operations in a great war to save the galaxy. The One appears on and off, independently unstuck in time, unidentifiable in a space suit. Everyone (except Zathras) gets off the station just in time.
The One saves Zathras, then takes off his helmet to reveal he's a much older Sinclair; he talks to an offscreen Delenn.
B plot: Delenn is summoned to the Grey Council, who say that she is their choice to replace their dead leader Dukat. Not wanting to give up her mission on Babylon 5, she refuses, and is removed from the Council, but receives an object, the triluminary, from one of her Council friends.
The Title Means: Probably just a fancy (but non-spoilery) way of saying "Two Babylons". Evocative of E=mc2 as well.
Comments: The plot summary there really doesn't touch on a lot of what happens, but a lot of what happens is really people just seeing and reacting to events going on around them outside their control, or lots of exposition. That doesn't make it a bad episode; it's just that there's not much to the central plot line of the episode (even if there's an awful lot of arc plot).
Last time we got an explanation for the planet we always saw in the background. This time we get an explanation for Babylon 4: we've heard repeatedly that the first two were sabatoged, the third blew up, and the fourth disappeared. So now we finally fill in that "disappearance".
Of course we don't get a lot of answers. Or rather, we get some solid answers, but first time viewers are still left with more questions than they started with. But I like how we do at least resolve the "The One" mystery, rather than leaving everything hanging.
Zathras is, of course, awesome.
Remember how I mentioned in a previous episode that the explanation for one of the mysteries was that (although we have not yet learned in it sequence) Morden was accompanied by invisible aliens? In this episode, I can see from the Lurker's Guide that some people watching it in real time became suspicious that there were invisible aliens in Garibaldi's flash forward, because they couldn't see who the shots were being fired at, etc. That never crossed my mind watching it already knowing there were invisible aliens (I just thought it was confusingly edited), so props to those folk back then. People also realized that Delenn must eventually go through some sort of transformation given the way she was hidden offscreen--a transformation that begins in only a few episodes.
The triluminary that Delenn receives in this episode is the crucial element for the machine she's been building in her quarters since Legacies. She seems astonished to get ahold of one, so it's not clear what she would have done without lucking into it here.
Let's look a little at Sinclair's flash forward. Because of cast changes, scuttlebutt on the internet is that JMS actually changed what the plan was. But he'd always intentionally had fallback plans for what he would do, so things are generally written with sufficient ambiguity to get away with it.
Now I will S P O I L several things, including the most complex twist in Babylon 5; you were warned!
Reportedly, the original plan was that Babylon 5 would be destroyed (as predicted by the seeress in Signs and Portents), and as seen in Garibaldi's flash-forward, and Babylon 4 would be brought forward in time to replace it in the upcoming shadow war. It's not clear if this is true since JMS apparently has denied this, and it would mean there were multiple time travel plots, but I believe JMS said at the beginning there was only one time travel plot in the whole thing. But it does make more sense for B4 to appear four years later if it's a stop on the way to the future.
What we actually get in the show that was filmed is that the two time travel plots are linked: Sinclair takes B4 back in time (instead of forward) to fight in a different great war: the war against the Shadows a thousand years ago. He will transform into a Minbari (the mechanism will be illustrated by Delenn soon) and become Valen, the great Minbari leader who has been spoken of reverently throughout the show by the Minbari, and who sets a lot of things in motion that will be necessary for events in B5. This also causes the intermingling of Minbari and human souls that, as we will see in a few episodes, is the reason why the Minbari surrended after capturing Sinclair in the Battle of the Line--to avoid hurting their own souls (the reality of souls being things, at least for Minbari, was established in the second episode).