Under Milk Wood presents a day in the life of a small Welsh town. The events described in the play are not given a specific date or time. Rather, the implication is that this is a day like any other for the residents of Llareggub, whose lives follow predictable routines. Consequently, there is no real plot in Under Milk Wood. The point of the play is to elucidate the cyclical nature of existence in the small town, and the meaning the characters derive from it.

Many of the residents of Llareggub take comfort in the routine nature of their lives. Dai Bread chastises his two wives when they do not have his food ready, as he is so dependent on his routine existence. Myfanwy Price and Mog Edwards are caught in their same romantic pattern, trapped by their fear of manifesting their love in anything other than letters. Each night, Jack Black puts on his special trousers and chases lovers through Milk Wood. He sees himself as an agent of good, chasing the immoral sinners from their den of iniquity. He feels comforted by the idea that he is—at the very least—doing something to encourage morality in the town, however ineffectual he might be.