The Promise of the Soul

When 69-year-old Cha Hsia's grandson announces his intention to undergo a sex change, Cha Hsia accidentally falls down the stairs with him, and his soul then enters the youthful body of his grandson. From that day on, he experiences many firsts in his grandson's body: going to college, living in a dormitory, being kissed by a man, and falling in love.
Does the classmate the grandson secretly admires actually like him back? In the dormitory, his grandson's boyfriend even takes off his pants and wants to… Who is the red thread of fate connected to?