Chief O'Brien returns from 20 years of virtual imprisonment
Chief O'Brien is visiting the planet Argratha, but when he displays his normal interest in their technology he is quickly arrested, convicted of espionage, and sentenced to twenty years of "virtual imprisonment." This fascinating concept means that while the Chief only spends a few hours in the corrections facility, his head is filled with two decades worth of memories of harsh confinement. So when O'Brien returns to DS9, he is a changed man who hoards food, sleeps on the floor, and refuses to talk to a counselor. "Hard Time" is an absolutely marvelous DS9 episode, featuring a bravura performance by Colm Meaney which stands up to that turned in my Patrick Stewart when he was tortured by the Cardassians. There is an emotional depth to this episode you rarely find in Star Trek, but which you greatly appreciate when you do.

The reason Colm Meaney moved from ST:TNG to DS9
This episode is one of the best of a very solid series. We all know that Colm Meaney has the acting chops but rarely is he showcased as beautifully as in this episode. He is allowed to carry this episode without very much interference from the other (admittedly steller) regulars. The scenes in the prison are tight, focused, and brilliantly acted. More than any other episode, this brings to light a new side of the character of O'Brien.

Absolutely, tragically beautiful.

Classic
This episode was quite simply a masterpiece. It centres on O'brien and his struggle to cope in extreme circumstances. Colm Meaney is brilliant as usual, the plot is simple yet involving and there are no easy answers or technobable reset buttons. Easily one of the best of season 4.