Babylon 5 awaits the arrival of Kosh Neranek, the Vorlon ambassador. The Vorlons are the most secretive of the four main alien bodies on the station, and as a result Dr. Kyle has only been given enough information to generate the correct atmosphere. The Jumpgate near Babylon 5 activates and Kosh's ship arrives, two days early. Sinclair sends Garibaldi to find Londo to ensure that he will be at the reception. Londo assures Garibaldi that he will attend, and Garibaldi leaves him to continue his gambling in the casino. After losing another bet, Londo is approached by a mysterious character called Del Varner, who offers him funds to continue his gambling.

G'Kar meets with the station's commercial telepath, Lyta Alexander, and offers her various incentives in an attempt to gain access to her genetic makeup. The Narn regime is the only main race not to have telepaths and G'Kar intends to re-create them using the genetics of a Human telepath. Sinclair steps into a lift to meet Kosh from his ship, but the lift fails and Sinclair is delayed. The Commander finally arrives in the docking bay to join Garibaldi and Lt. Cdr. Takashima. Shortly after, an alarm sounds and Sinclair sends Takashima off to alert security. Sinclair opens the door to the docking area to find Kosh on the floor, and Dr. Kyle is instructed to get MedLab ready for the Vorlons' arrival.

Once in MedLab, Takashima explains that the Vorlons will not allow anyone to open his encounter suit. Sinclair orders the MedLab recorders to be turned off and tells Dr. Kyle to do what ever he can to save the Vorlon. Kyle proceeds to open Kosh's suit to try and locate the cause of his illness. Garibaldi is given full diplomatic access in an attempt to find out where people were at the time of Kosh's attack. Kyle wakes the Commander to inform him that Kosh was poisoned and Sinclair seals of the station.

With the Vorlons' instructions to turn off all the cameras in the docking bay it is decided that only Kosh himself knows who attacked him and they ask Lyta to illegally scan him. Lyta is unhappy with the request, but performs the scan and discovers what appears to be Sinclair poisoning Kosh. Word gets out and Sinclair is forced to surrender control of Babylon 5 to Takashima.

The Advisory Council is called and votes to deport the Commander to the Vorlon Homeworld to stand trial. It soon becomes apparent that someone on board has the ability to change their appearance, as Dr. Kyle is attacked by an assassin disguised as Lyta in an attempt to kill Kosh. Garibaldi sets out to find Del Varner, the man who delayed Londo from the docking bay, and discovers him dead in his quarters. He also discovers that someone gained access onto Babylon 5 through a pod found attached to the outside of the station.

Takashima takes a look through Del Varner's files and finds that he was carrying a Changeling Net, an illegal device that can change the voice and appearance of someone. With this information the crew decide that someone is trying to frame Sinclair and knowing that the device will give off an energy field the scanners are set to look within Babylon 5. With the assassins location now known, Sinclair and Garibaldi move in to capture it.

The Vorlons are now demanding the immediate transportation of Sinclair and threaten to attack in 5 minutes with devastating force. Sinclair is attackedand during the fight the Changeling Net fails and the attacker is revealed to be a Minbari. Sinclair manages to kill the assassin but as it dies it tells him that "There is a hole in your mind". Sinclair realises that the assassin is referring to the 24 hours on the Battle of the Line that he cannot remember. The Minbari activates a self destruct device and Sinclair is forced to run to avoid the explosion. Later Sinclair confronts G'Kar about his involvement in the situation. He plants a fake homing device in G'Kars drink and warns him about further problems. The reception to mark the arrival of Ambassador Kosh is finally held and the assembly of the main races aboard the station is completed. Later, Delenn finds Sinclair alone in the gardens and Sinclair asks her about the 24 hours of his life that he cannot remember, but she reveals nothing.