The populace welcomes the return of victorious Crispo, son of Emperor Constantine and his first wife Minervina. A high priest by the altar is waiting to crown Crispo who has been successful in his campaign against the Gauls. Fausta, the second wife of Constantine, and her father Massimiano (archrival of the Emperor), as well as Licinia, Constantine himself and scores of prisoners witness the triumph of Crispo. Fausta looks at him and feels the pangs of her incestuous love; Constantine rejoices and Beroe, one the prisoners, remembers her love at first sight for Crispo. Constantine notices her and wonders about the female prisoner; Crispo explains that she is the daughter of a prince and declares his love for her. Fausta is stunned by the development and the Emperor tells Beroe to come forward; she declares her love and Constantine tells her that he is going to marry them that very day. Fausta is devastated by the announcement and tells her husband that the wedding will have to be set for the following day because the goddess Vesta so requires. Licinia and those attending the event rejoice. After the jubilation, everyone leaves except Massimiano who is conspiring against the Emperor, and although he failed his first attempt to kill both father and son, this time he is sure he will succeed.