DS9 Season 4 Episode 19 Hard Time

Plot

Chief O'Brien is unjustly convicted of espionage on the planet Argratha. Instead of incarcerating convicts, the Argrathi correctional facility implants them with memories of years of imprisonment in a few hours of actual time. O'Brien experiences twenty years in prison before his fellow officers can extract him from the situation.
On returning to Deep Space Nine, O'Brien tries to adjust back to life on board the station, but his experiences while imprisoned still trouble him. Although he tells Dr. Bashir that he was alone in prison, flashbacks reveal to the audience that he had a cellmate, Ee'char, who taught him how to survive while incarcerated. He begins to exhibit habitual behaviours that he "picked up" while "incarcerated," such as hoarding food and sleeping on the hard floor instead of his bed. He begins to experience hallucinations of Ee'char.
Over time, O'Brien becomes increasingly irritable and reclusive. He has an altercation with Bashir, and assaults Quark when he has to wait to be served at Quark's bar. Captain Sisko later relieves O'Brien of duty, placing him on medical leave and ordering him to attend the counselling sessions that he has been avoiding. After he returns to his quarters, he almost hits his daughter Molly when she tries to get his attention. He flees to a cargo bay and starts tearing the place apart. Finding a weapons locker, he sets a phaser on maximum, preparing to take his own life.
Bashir follows O'Brien and finds him just in time. O'Brien tells Bashir how he killed Ee'char over a few pieces of bread that Ee'char was saving for the both of them, and that the guilt has been eating away at his conscience. O'Brien is convinced he is an animal, but Bashir tells him that if he truly were, he would have felt no regret for killing Ee'char, and so O'Brien is still human deep down inside. Bashir ultimately prescribes a new course of heavy treatment and soon has O'Brien on the way to recovery.