Junie B. Jones is Captain Field Day is the sixteenth book in the Junie B. Jones series.
As the captain of Room Nine's field day team, Junie B. tries to rally her troops after they lose several events.
Being the anticipated day of her Field Day at afternoon kindergarten, Junie B. wakes up ecstatic and excitedly wakes up her baby brother Ollie, causing him to scream. Junie B.'s mother enters the room, as Junie B. once again proclaims it is Field Day. After being confronted by her mother, Junie B. calls one of her best friends, Grace, on the phone. After mistaking Grace's dad's voice for her own, Junie B. is relieved Grace is not sick as she is the fastest runner in Room Nine. After a minor argument about who is faster, they settle their differences and hang up, starting their day with breakfast.
Later that day, at Room Nine, Mrs., Junie B.'s teacher, scolds her, Grace, and Lucille for warming up by skipping and dashing around the room. Mrs. reassures them they will have plenty of time to warm up outside and announces to the class they will pick a captain for their team, she explains, by having each student choose a slip of paper and the one with the word "CAPTAIN" nominates the student as Room Nine's captain. After stalling for quite a bit, Junie B. picks a slip of paper, hoping no one else receives the winning paper, and discovers she choose the one with wording on it. Excited she is selected as team captain, she announces to the class, but to her surprise, no one else is excited with her.
It turns out Junie B. is mistaken for a captain being the same thing as the boss, or a superhero. She fantasizes about her new outfit, then becomes disappointed because Mrs. reveals the team captain is supposed to encourage the members of his or her team, not boss them around. Finally, her teacher agrees to let her wear a red towel, which she settles for as a cape. Finally, Mrs. takes her pupils in a single file line (with Junie B. once again reminding the class she is their team captain) to begin Field Day.
While outside, Junie B. is surprised to be informed that New Thelma, a girl who transferred to her school and then resulted in Ricardo dumping Junie B., is the team captain of Room Eight. Junie B. is reluctant to shake hands with New Thelma, remembering her grudge against her, until Mrs. tells her she must shake hands with New Thelma and wish her team good luck as it is good sportsmanship of the team captain. After pettily shaking Thelma's hand, she is once again surprised that New Thelma remembers her as well, and that her boyfriend used to be Ricardo. After wishing Room Eight's team good luck, Junie B. hears her grandpa Miller call out her name. She turns around and sees her parents and her grandpa and grandma, and Grandpa Miller spins her in the air like she is a real superhero.
Junie B. then hears Mrs.'s whistle and darts back to her team, triumphant of winning every event. The Room Eight teacher announces that since there are eighteen students present in both classes (making a total of 36 kindergarteners in all), every student will get an opportunity to participate in the relay race. After Mrs. creates a line in the grass where the runners would line up and explains the rules, Junie B. assures Grace that because she is the fast runner in kindergarten, she needs to go in front, but to her dismay discovers a girl named Charlotte is already first. However, after a bicker argument between her and Junie B., Grace convinces Charlotte to let her cut in line (to Junie B.'s surprise until she explains she simply said "please") and Mrs. prepares the teams for the team relay race. After the countdown, Grace runs (as fast as a rocket, according to Junie B.) until she reaches the fence, zooms back to her teammates, and taps Charlotte on the hand. After several other students in Room Nine run, they are ahead of Room Eight. Finally there are only three more runners in Room Nine: Ricardo, Junie B., and a boy she calls "Crybaby" William, because he is always so nervous.
As Ricardo starts running to the fence and back, William nervously tells Junie B. he is not a very fast runner, but is reassured by Junie B. she will run so fast as team captain, he will barely have to run. When Junie B. is tagged by Ricardo, she immediately dashes to the fence. Unfortunately for Room Nine, on the way back, Junie B.'s shoe flew off her sock, and she stops to reposition it on her left foot. As she taps William on the hand, strangely, he stays in his one spot. After Junie B. frantically shouts for him to move it, William gestures to Room Eight, already celebrating because they won the relay race event.
Room Nine treats Junie B. poorly after the relay race, as Junie B. asks if she was expected to run with one of her shoes missing. A bully named Jim explains angrily it was exactly what she was supposed to do. Junie B. nervously confronts Mrs., informing her everyone is blaming her for the loss of the first event.
- This is the last book where Junie B. is 5 years old.