"The Last Summer of Youth" is the final major Penny and Aggie story arc and, at four chapters and a running time of nine months, the second-longest arc in the series. During its course, the romantic undercurrent in the title characters' relationship, hinted at throughout much of the comic, finally comes to the surface, as Penny and Aggie explore being more than friends. The relationships, romantic and otherwise, of several supporting characters also undergo challenges and changes.