I understand that the requirements for a plenary indulgence include confession. Is the requirement for confession contingent on having serious sin, or do you need to go in any case?
The Apostolic Constitution on Indulgences states that sacramental confession is required to acquire a plenary indulgence. There is no exception for one who is not conscious of mortal sin:
To acquire a plenary indulgence, it is necessary to perform the work to which the indulgence is attached and to fulfill three conditions: sacramental confession, eucharistic Communion, and prayer for the intentions of the supreme pontiff. It is further required that all attachment to sin, even to venial sin, be absent. If this disposition is in any way less than complete, or if the prescribed three conditions are not fulfilled, the indulgence will be only partial. (ACI 7)