MC is a dude who had a good job but quit to trade stocks. Lost everything and is now living with his dad and his new wife who is the same age as him. Step-mom's sister (the FMC, and by virtue of being step-mom's sister, she is the "Aunt" of the title) also moves in to 'study' for entrance exams. MC has the hots for his step-mom from the get go and makes his move on her with no qualms at all. Under the pretext of 'wanting to get along with her new step-son' she allows him liberties which soon morph into a full blown, no holds barred relationship. MC's father is a cartoon. MC is made in-charge of Aunt's test prep while she also goes to a tuition center where, surprise, there is MC's old classmate who has the hots for him. Aunt is bratty to the MC but then succumbs. He also makes it with his ex-classmate, but then turns her down when she proposes. The two sisters figure out that they both love MC and are delighted about it. MC moves out of the house, finds a job. They all stay apart one year and FMC works hard and passes her exams and the threesome resume their shenanigans.