Swallowtails unfolds over the course of a late night outside a 1983 prom, following two cousins drifting at the edges of the same moment. Each is reeling from their own version of rejection and holding very different views on life, but as the night stretches on, distance gives way to understanding. When one is confronted with her boyfriend’s betrayal, she finds the courage to stand up for herself—transforming both her sense of self and their relationship.