Mizuiro Jidai is the story of the life of Kawai Yuko, a normal girl growing up. The series begins at the start of her 8th grade year of school, and follows her life through its many small and large ups and downs. The anime's title refers to the time in a girl's life when everything is new, a time of growing up.
Yuko is a cute, somewhat insecure girl. She is a good writer but is somewhat shy of her talents and worries about others not liking her work. Yuko has grown up all her life in the same house with her mother, father, and older sister. Living right next door is one of Yuko's closest friends, Hiroshi. Yuko and Hiroshi have been friends since childhood, but recently their feelings for each other have been changing.
The series gives the audience a view at Yuko's life through the seasons as she goes through such events as school trips, jyuku (cram school), band, making new friends, finding love, dealing with her parents and learning more about them, holidays, and sports. Yuko's normal fears of the future and the unknown are explored, and the joys of her life are shared.