The speaker catches a huge fish while fishing in a little rented boat. She studies her catch for a while as, holding it up half out of water beside the boat. The fish is pretty old and gnarly-looking, with barnacles and algae growing on it, and it also has five fishing hooks with the lines still partially attached hanging from its jaw.
The speaker considered how tough this fish must be and how much he probably had to fight. She begins to respect the fish. The poem takes its final turn when the oil spillage in the boat makes a rainbow and the speaker, overcome with emotion by the fish and the scene, lets the fish go.