Day & Night follows two anthropomorphic characters, Day and Night. Inside Day is a day scene with a sun in the center, and inside Night is a night scene with a moon in the center. Whatever goes on inside of Day or Night expresses normal events that typically occur within a day or night, respectively, and these events often correspond with actions or emotions that the characters Day or Night express. For example, when Day is happy he will have a rainbow inside him, and when Night is happy he will have fireworks inside him.
Day and Night meet and at first are uneasy about each other. They become jealous of each other due to the events occurring in their insides, and end up fighting at one point in the short. Eventually they see the positives in each other and learn to like each other. At the end of the film they see the things they saw in each other in themselves, as Day becomes night, and Night becomes day.