The speaker begins "The Fish" by reminiscing about a fish she once caught. This fish does not struggle as she reels him in, and once he is captured, she begins to take special note of his appearance.

He is a large fish, hanging with a "grunting weight" from her rod. His scales are no longer those of a young fish. They "hang" from his body instead of conforming to it. They remind her of old wallpaper, which loosens from the wall and whose patterns fade with age.

The fish is physically unappealing. He is covered in barnacles and sea lice. Some green weeds dangle from his body, likely remnants of the home where he rested before her hook held fast in the corner of his mouth.

She watches the fish breathe in the "terrible" oxygen, described as such because this very act is taking his life, one breath at a time. His gills, which are "fresh and crisp with blood," long for the water which they need to sustain his life.

The speaker then looks into the fish's eyes, which also reflect his advanced age. They have yellowed over time, and they remind her of looking at tinfoil through "old scratched isinglass." Although his eyes shift, they do not seek to meet her gaze, the gaze of the one who has taken him from his natural environment and seeks to take his life.

She then shifts her focus to his jaw and notes that from his lip dangle "five old pieces of fish-line." The fish has been caught and escaped at least five times prior to this day. She sees these lines like medals, showing evidence of battles this fish has fought and won. She reflects on the fish's "wisdom" in overcoming so many attempts on his life.

And suddenly, her perception shifts. This is a victorious fish. He has lived through so much in his old age. The feeling of victory fills up her entire boat, and she notes the rainbow, often symbolically representing brilliance and life life, in the oil residue in the water. She knows that she cannot be the one to take the life of an old fish who has overcome so much.

And she honors the fish by letting him go.