Senju Tobirama is a second-generation Hokage who is serious in style, tough and pragmatic, holds real power, and has a high reputation. He is single now. Judging from his behavior, he is ready to dedicate his limited life to the infinite cause of fighting for Konoha.
Uchiha Kage is a young talent who is highly expected to be the future heir of the Uchiha clan. He is a direct disciple of the Second Hokage. He is gentle and intelligent, and has great experience in observing words and expressions.
This setting was supposed to be a master-disciple story between Senju and Uchiha. The only surprise was that Kagami fell in love with his teacher.