Meursault is in prison awaiting trial. He dispassionately dislikes it, and decides that, should he get out, this is one phase of his life he probably wouldn’t enjoy talking about. ("Hey baby, I just got out of prison" isn't likely to be a good pick-up line.)
Marie visits him, full of smiles, in a room without privacy; everyone has to shout to be heard. There is a certain kind of dizziness to the visitation room. Meursault wants to tell her she looks beautiful, but he "doesn’t know how."
Marie shouts to Meursault that he has to have hope. Meursault says yes, but only because at this moment he wants to squeeze Marie’s shoulders and feel the thin material of her dress. He watches the obvious emotion in the connections formed around him, between other prisoners and visitors; it seems totally alien to him.
Marie shouts again that they’ll get married one he’s released. Meursault doubts this, but responds with a useless "You think so?"
Meursault has to force a smile at the end of the visit, as he is led away.
After that, Meursault receives a letter from Marie stating that she is not allowed to visit him further, as she is not his wife.
Meursault’s biggest problem in the first few months of prison is that he had the thoughts of a free man. Often he’d suddenly experience the urge to walk on the beach and swim in the water. Or he would think of women. Never Marie specifically, but just any and all women.
Meursault makes friends with a guard who tells him that’s the chief complaint among prisoners. But, he wisely states, that is also why you’re in prison.
After that, Meursault’s thoughts became those of a prisoner. He gets used to life as an inmate. He isn’t too unhappy, but more just annoyed. Besides, Meursault reasons, a person could get used to anything after a while.
One game Meursault plays to pass the time is to focus on an object, like a newspaper story, and try to recall every excruciating detail about it. He realizes from this that a man who has lived only one day could easily live for a hundred years in prison – he would have enough memories to keep him occupied. Sex, not so much, but memories, sure.
Meursault also sleeps a lot, kind of like his non-prison days, because it helps passing and losing all sense of time. His days end up flowing into one another, like when you take a nap at three p.m. and wake up thinking it’s Tuesday, even though you fell asleep on Sunday and besides, it’s still light out.
Meursault realizes that he has grown serious… morbid… joyless. Which is totally different from before. Maybe.