This is the first episode that deals heavily with Sinclair's backstory and, it will later turn out, the main plot, and it's a weird one.
It all starts out fairly straightforward, with Sinclair and Garibaldi questioning one of the security personnel, Bensen, about his gambling habits. They believe he's been placing bets over the limits imposed on station staff and, although Bensen denies it, they're right. In order to settle his debts he supplies a power cell to a couple of shady characters.
In the meantime, Doctor Franklin is examining Ambassador Delenn to gather data on healthy Minbari. Since he usually sees them when they're sick, it will provide him with a baseline to work from. They chat about his past hitchhiking on starships and working for his passage, and that during the Earth-Minbari war he destroyed his xenobiology notes rather than have them used for biological warfare. Delenn is surprised and grateful, and dodges his question as to what she did during the war.
That night, Sinclair wakes up after a nightmare of the Battle of the Line. He finds his comms not working, and in one of the series more unsettling scenes he makes his way to C&C to find it - and the whole station - deserted. All except for one other person. His unnamed captor tells him they're inside Sinclair's mind, in a virtual reality constructed to get to the bottom of what exactly happened during Sinclair's missing 24 hours during the battle - at the end of which the Minbari surrendered. Sinclair's report stated his ship was damaged, he tried to ram a Minbari cruiser and blacked out, but there are a number of people back home who don't believe him. They think he betrayed Earth, agreed to be a spy for the Minbari, and that the increasing alien presence and influence back on Earth is evidence of this.
The interloper tries a number of tricks to get Sinclair to cooperate. He summons and kills an image of Garibaldi, inflicts pain on Sinclair, and conjures shades of a dead comrade who accuses him of betrayal. Sinclair, realising that time is limited and people will be looking for him, fights back and hurts his captor, forcing him out of the simulation.
While all this could be happening at the speed of thought, it actually takes long enough that Chief Garibaldi reports Sinclair missing - he's an hour late for a meeting and no one's seen him since the previous evening. He starts a full scale search of the station, even outside in case they're looking for a body. The body they find is Bensen, who interrupted the interrogation of Sinclair and is killed for it. The search's proximity to where Sinclair's being held makes his kidnappers panic and they ramp up the psychotropic drugs in the hope it will make him snap.
It does, but not in the way they expect.
Sinclair remembers the Minbari assassin in the pilot telling him "there's a hole in your mind". He remembers ramming the cruiser, being taken on board and inspected by the Grey Council who wave a glowing device at him. He remembers seeing Delenn there. The shock of it ejects him from the virtual reality where, believing himself in hostile territory, he smashes the simulator. He then tries to kill Garibaldi, who's just found him. Hallucinating and frightened, he goes on on the run.
What follows is a three way chase with the second captor chasing Sinclair, Garibaldi's forces chasing them both, and Sinclair unable to tell friend from foe. He's stopped by Delenn, who puts herself in the line of fire to talk him down. He comes out of it enough to shoot the second captor before collapsing and being taken to medlab.
The plot here is a fairly simple one, the main story being Sinclair's experiences and the B plot the search for him. But the episode overall does some pretty hard work in terms of characterisation and story arc. As well as finding out more about Doctor Franklin, it also reveals more about Sinclair. The mental construct is of Babylon 5, and that's where he comes back to when resisting his captor, who accuses him of hiding behind duty and responsibility. We see a triluminary for the first time, although we're not told what it is or what the Grey Council are using it for. It's also revealed that the growing xenophobia back on Earth is deeply rooted, possibly even within the government.
At the end of the episode, Sinclair goes to see Delenn to thank her for her help. He tells her "strange that seeing you snapped me out of it" and it's a nice touch that it's not clear which time he means. She asks if he remembers anything of his experience - another ambiguous phrasing, since it's not clear here either which encounter she's referring to. Sinclair lies and tells her no, although it's suggested Delenn suspects he's lying simply in the way she reacts. After he's gone another member of the Grey Council warns her that should he remember, Sinclair must be killed. In his quarters, Sinclair admits in his log he remembers. His goal now is to find out what the Minbari wanted with him ten years ago.