A story about a family. Due to adult circumstances, the son, Masaru, temporarily stays at his aunt's house, where he unexpectedly finds himself drawn to his mother's presence. Deeply moved by his mother's beauty, his romantic feelings gradually shift toward her. This internal conflict and his complex relationship with his beautiful yet strict mother are portrayed in the form of B·I·R·E·I Adulterous Sex. This work carefully depicts the son's emotions, the bond between mother and child, and the whirlwind of feelings that emerge between them.