"It was the middle of the Earth year 2260," recalls Sheridan. "The Year Between Wars, and the beginning of a new age. The Shadow War was over, but there was still a darkness waiting for us back home. President Santiago had been assassinated, and his successor, president Clark, was turning Earth into a prison camp. We'd broken away from Earth, and in retaliation they had quarantined us, trying to strangle our supply lines. We were desperate, and we couldn't afford to lose even a single supply ship. In the midst of all of this, we made contact with Thirdspace, and we almost didn't survive it... I only hope we never come across it again. Because the next time, no one is going to survive it."
A supply ship is under attack by raiders, and the raiders are under attack by Starfuries, led by Ivanova. Ivanova tells the raider leader that their forces are too evenly matched, and it will be at best a stalemate. But the ship carries Quantium 40, too rich a prize for the raiders to ignore. So they call in their heavy cruiser, which functions as their base. And then, Ivanova springs her mouse trap: Sheridan, in command of several White Star ships, arrives and takes care of the cruiser. Hopelessly outnumbered and outgunned, the raiders surrender. Sheridan takes the prisoners to a Drazi freeport, and Ivanova takes the wing of Starfuries back to B5. She'll spread the news; hopefully, if raiders believe any shipment could be a trap, they will think twice before attacking.
Back on B5, Lyta Alexander is mediating a private dispute, but she keeps getting flashes of a ship or artifact in hyperspace. It discomforts her greatly, and quickly excuses herself. Meanwhile, in hyperspace, Ivanova's wing detects something lost there, adrift. They make a lifeline of Starfuries to reach it before it drifts away and is lost ("A Distant Star"). It is a huge construct, of unknown origin. Ivanova requests Babylon 5 send White Stars and all available starfuries to tow the enormous find back to the station.
As the artifact passes through the gate, Lyta has a vision: the Zocalo in ruins, bodies everywhere, herself hurt and bleeding, and a strange shadow stalking her...
Later, Vir talks with Sheridan and Delenn. Londo is away on Centauri Prime, and the League has asked him to talk on their behalf. The League want access to the artifact, and the possible advanced technologies it represents. But Sheridan is wary of having everyone poking at the artifact, pulling in all directions at once. He promises they will share anything they find with the League. Vir does not think the League will be satisfied with promises, but agrees to relay Sheridan's answer to them.
But as Delenn points out, Babylon 5 is not equipped to handle research of such proportions. "So it will take us a little longer," answers Sheridan possessively.
A maintenance bot is sent to take a surface sample from the artifact, but its energy is drained, and strange markings of some kind shine briefly on the artifact's surface. And in her quarters, Lyta is furiously scribbling on the walls "There is danger. Remember." over and over again...
Soon after, a ship from InterPlanetary Expeditions (IPX) arrives and requests a meeting with Sheridan. The leader of the expedition is Dr. Elizabeth Trent. Despite the Earth embargo on B5, IPX would like access to the artifact. Because of its line of work, IPX maintains contacts with other worlds and outside of Earth-controlled space, and Earthgov turns a blind eye towards them for business' sake. Dr. Trent points out that B5 is not set up to investigate the find, and it could even be dangerous to be poking around blindly. Sheridan agrees to given them access, provided IPX meets some conditions: they are to keep him and the station informed at all times, and give them full access to their finds; he also wants first shot at any new technologies found; and anything that could be used by Earth against the other races will not leave B5; Sheridan is willing to destroy the artifact if necessary to prevent such information from leaving. Finally, since IPX can come and go as they please, they will bring needed supplies and foodstuffs to the station. "If you're going to work here," explains Sheridan, "you are going to have to bring lunch. I just want to make sure you bring enough for everybody."
Dr. Trent wants to think about it, but just then Ivanova links in: their tests suggest the artifact is at least a million years old. This is more than enough for Trent, who agrees to the deal Sheridan proposed.
In her quarters, Lyta is dreaming of the artifact. In her dream, it is covered in some sort of writing. She wakes up and leaves. As she is going up on a lift, she is joined by Zack. She, however, is still lost in her visions, mouthing the words "there is danger, remember" over and over. The lift stops suddenly: the station has been experiencing power fluctuations for the past couple of hours. Zack, meanwhile, uses the pause to ask Lyta out; he is taken aback by her silence and detachment, thinking it is rejection. He eventually backs off, and requests that the whole conversation be forgotten.
Later, as more work continues around the artifact, dozens of maintbots accelerate towards it on a collision course. They are destroyed by starfuries before they impact, and when security breaks into Maintenance, they find Lyta standing over the unconscious bodies of the personnel. "We have to stop it, we have to stop it," she says, and falls to the ground unconscious. Later, she tells Dr. Franklin she cannot remember anything since the previous evening. Although Dr. Trent wants her in custody, Sheridan refuses. He will treat this as an isolated incident.
The preparations around the artifact are complete, and the IPX team begins deep magnetic scans on the artifact. They figure it is not a ship, but rather some sort of mechanism. Dr. Trent even has some ideas about what it is...
In the Zocalo, a man starts dreaming of a city in space, with a spire in the middle, and dark tentacles reaching for him. He awakes screaming. All over the station, similar events are occurring: fights are breaking out, people are locking themselves up in their rooms, screaming...
In Downbelow, Deuce ("Grail") is looking up; he is seeing the same city, and mutters to himself "It is beautiful." Ivanova also dreams of the city, and even sees Vir there. A dark tentacle reaches over from behind her, and she wakes up screaming.
In the morning, Bill Marishi, the second in command of the IPX group, is working over breakfast. Suddenly, he is surrounded by silent people who are staring at him intently. Among them is Deuce, who tells him IPX isn't moving fast enough. "You have to move faster," he tells Marishi. "You have to make it work."
Despite her progress, Dr. Trent has not shared her findings with Sheridan, and he is getting suspicious. Although he is not a xenoarchaelogist, from his years with Anna he knows they should be further along than Dr. Trent is letting on. But Dr. Trent claims the situation is unique and calls for slow progress. Sheridan then notices some of her notes, and recognizes that the writing on the surface of the artifact is Vorlon. Dr. Trent should know that, and moreover should have shared the information. Sheridan gives her 48 hours to get a report to him, otherwise he'll pull the plug.
Faced with the deadline, Trent tries to push up the schedule. She wants to crack the artifact's technology before anyone else takes it away from her. But Marishi disagrees, and since he is second in command his agreement is needed. So she reveals what she knows: the way Dr. Trent figures it, the artifact will open a gateway to another space, different from both normal and hyper; she calls it "thirdspace." As far as she can tell, it operates under a whole different set of rules: a ship traveling through thirdspace would be able to travel almost instantanously between points that would take days or weeks through hyperspace; the technological, and more importantly the military, potential is enormous. Although cautious, Marishi agrees to go ahead and try to turn the artifact on, and open a gateway to thirdspace.
In a lift, Ivanova meets Vir. Since it is considered good luck among Centauri, Vir mentions he saw her in his dreams last night. Soon, Ivanova realizes it was the same dream she had, and that this is connected to the strange happenings all around the station, and to the artifact itself. She rushes to tell Sheridan about it.
But Trent has put her plans in action. The work crew plugs in a power cell to the artifact, which springs to life.
People start converging on the observation dome, the Zocalo, and the cobra bays, entranced by the artifact. Some are trying to stop the starfuries from launching. When Zack tries to clear them out, fights erupt. The artifact is now draining power from the whole station. Zack links in to Sheridan: fights are breaking out everywhere, and people seem to be under some kind of telepathic influence or trance; he won't fire on them, which makes things that much harder. Some starfuries launch and start firing at the artifact from a distance, but the artifact seems to be protected by an energy shield now. Slowly, the artifact begins to open...
Sheridan goes to see Lyta, and is met by Delenn. They find Lyta taken over by a presence, an "echo of what was." The presence explains what the artifact is: thousands of years ago, the Vorlons, believing themselves to be just short of godhood, constructed the artifact as a gate to heaven. In their arrogance they didn't stop to consider the consequences and dangers. On the other side, they found a powerful race bent on the destruction of all other life in that universe. With the gate open, they came into our universe to do the same. Strong telepaths, they even managed to take over some Vorlons. The Vorlons managed to fight them back and close the gate, but before the artifact could be destroyed, those who had been taken over flung it into hyperspace. Now the door is opening again, and the artifact must be destroyed. For the aliens to come through, the door must be open on both sides, so this will suffice. The presence imparts to Sheridan the knowledge needed to destroy the artifact.
Outside, the gate is opening and small ships are coming through. They easily take care of the few starfuries that have launched...
Inside, Sheridan is ready to go ahead. Delenn will get some Minbari heavy cruisers, and Sheridan has a mission of his own. Ivanova goes to see Trent, and finds Bill Marishi dead, shot from the back: Dr. Trent has been taken over as well. Before she can strike again, Ivanova disarms her and knocks her unconscious. She then leaves to lead the arriving White Stars, as more alien fighters come through the gate.
The alien fighters are very powerful and tough, and even the White Stars and the Minbari cruisers are having trouble holding their own. Meanwhile, Sheridan goes to the armory and takes a tactical nuke, before going out on a powered spacesuit. The defending fleet concentrates its fire on the front of the artifact, opening up a hole in the defensive shield on the back. Sheridan slips through it to the inside of the artifact.
As the fights rage on outside and inside the station, Sheridan places the tactical nuke in the center of the artifact. He arms it, and on his way out he finds his way blocked by creatures which look like those in Ivanova's and Lyta's visions. He manages to escape them barely, as a giant alien ship starts coming through the gate. Before it can emerge, however, the bomb goes off, destroying the artifact and the alien ship.
The combined defenders slowly and painfully destroy the remaining alien fighters, as the people inside Babylon 5 slowly awaken from their telepathic takeover. The station, and the universe, have survived.
Later, Dr. Trent is apologizing for her behavior, not all of which was telepathically induced. She acknowledges she was wrong and will take a leave of absence to take stock of her life. "I didn't like the part of me that thing brought out," she tells Sheridan. She also gives him all the information she found about the artifact. It may prove useful, some day.
"I was sure she didn't give me all the information," remembers Sheridan. "She just wanted out of there. And if a little lie got her out the door, that was fine with her. Fine with both of us, really. Because as Mr. Garibaldi has been known to say from time to time, `we all lie.' When the dust settled and people wanted explanations, we gave them one. We said that the device was a weapon, a mouse trap, which was true. We told them that it used a telepathic trigger and caused an outbreak of fighting, and we told them that it was finally destroyed. It's all true.
"And of course, it's all a lie. Because there are times we don't need the whole truth. The Vorlons made an error thousands of years ago, and we paid the price for it today. Why encourage any one else to do the same? No, we have more than enough troubles of our own, we don't need to inherit anyone else's mistakes. We got through it alive, and everyone's okay, and that's the important thing. And this isn't the kind of problem that's likely to ever happen again."
In the Observation Deck, Lyta is staring sadly at the stars, saying to herself: "One mistake. One mistake out of so many, so many others..."