Description

 The film deals with an enormous artifact that is discovered in hyperspace and towed to the Babylon 5 station for investigation, at which point the xenoarchaeology corporation Interplanetary Expeditions sends a representative, Dr. Elizabeth Trent, to take control of the artifact's examination.
 After being placed near the station the artifact begins to influence the dreams of many inhabitants of Babylon 5, eventually controlling many of them during their waking hours as well. These thralls, led by Deuce, first demand that the excavation be accelerated, and then become increasingly violent towards the rest of the Babylon 5 population.
 Eventually it is revealed by Lyta Alexander that the artifact is a Jumpgate that takes one neither to normal space nor to hyperspace but to a "third" space (hence the movie's title), built by the Vorlons a million years ago with a purpose that cannot be expressed in human terms except as an attempt to make contact with the gods. In reality, Thirdspace is inhabited by a violent species that posed a threat even to the Vorlons and other First Ones. The ensuing battle ended with a group of Vorlons controlled by the Thirdspace Aliens capturing the artifact and jettisoning it into hyperspace, in hopes that it might one day be reawakened.
 When the device is finally reactivated, the Thirdspace Aliens stream out and begin an assault on Babylon 5. The violent behaviour of the individuals under the artifact's control is intended to disrupt the station's defenses. The struggle is ended when John Sheridan enters the artifact to plant and later detonate a nuclear bomb.
 The central theme of Thirdspace is hubris. The artifact was created because of the Vorlons' belief that they were equal in power to the gods, perhaps a reference to the biblical story of the Tower of Babel, and it is as a result of Sheridan's and Trent's shared pride and refusal to cooperate that it is activated.