A huge artifact is discovered in hyperspace and towed to Babylon Five. The xenoarchaeology corporation Interplanetary Expeditions sends Doctor Elizabeth Trent (Shari Belafonte) to examine it. The artifact influences the dreams of Babylon Five’s residents until it’s controlling the daytime activities of many. They are called Thralls and a Lurker named Deuce (William Sanderson) demands that the excavation be speeded up, as people become violent toward each other. Doctor Trent believes the artifact is a jump gate to a third space—neither hyperspace nor normal space. Trent helps to turn on the device without checking with Captain Sheridan (Bruce Boxleitner).

Sheridan and Delenn (Mira Furlan) recruit telepath Lyta Alexander (Patricia Tallman) who determines that the Vorlons built it a thousand years ago to contact the gods. Thirdspace is a parallel universe inhabited by telepathic aliens even older and more powerful than the Vorlons. Long ago, they converted an army of Vorlons to fight for them. In the end, the non-thrall Vorlons shut down the gate, but the thralls sent the artifact into hyperspace to hide it. Lyta telepathically informs Captain Sheridan how to deactivate the gate.

Thirdspace allies stream out in small fighters protected by deflection shields, and attack Babylon Five. They can destroy large cruisers with ease. The thralls on the station try to disrupt Babylon Five’s defenses. Sheridan uses a thermonuclear device to destroy the artifact, just as a wave of heavy warships emerges from thirdspace. The telepathic influence is cut off and the station returns to normal. Doctor Trent turns in her report and goes on hiatus. Sheridan conceals the true nature of the artifact, but Lyta wonders what other mistakes the Vorlons made.

The hubris of the Vorlons in thinking they were gods is a reference to the story of the Tower of Babel. The episode is set just after the Shadow War and aired in the middle of the fourth season. In a scene written because they needed a few minutes more of airtime, Zack Allan confesses his attraction to Lyta, not knowing that she is telepathically dominated and not listening. The device was partly designed by Douglas Barlow, author of Barlow’s Guide to Extraterrestrials. The Vorlons referred to the thirdspace aliens as Harbingers. They are seriously creepy and remind me of something out of the Cthulhu mythology of H.P. Lovecraft.