The Kurusu family's homestay grows to three in the blink of an eye. Not yet accustomed to living with three monster girls, Kimihito's life is a series of disasters. When Miia sees Kimihito helping others, her jealousy boils over. With each girl loving Kimihito in their own way, they eventually come to blows!

While undergoing physical examinations, which include bust-waist-hip measurements, most of the girls find out they have been gaining weight, but they can't go out to exercise without Kimihito. Smith sends Kimihito and the girls to a gym suited for Monster Girls, owned and run by a kobold named Polt. She takes everyone on a tour of the brand-new facility, but finds out that not everyone is suited to or able to use the exercise equipment. A visit to the gym's oversized pool leads to a race between Miia and Mero, with Kimihito as the Grand Prize.

One morning, the daughter of Rachnera's previous host family, Ren Kunanzuki, shows up with a petition for Rachnera to return to them. However, Ren is quite obsessed with her stubborn attitude that taking in Rachnera constitutes a trial for her patience and capability; and her encounters with Kimihito's other housemates only serve to strengthen her opinion until Kimihito finally returns home. In a post-credit skit, Miia persuades her fellow housemates and the girls of MON to try on new bras and pantyhose she has purchased for them all.