Naive T Cell's patrolling an abandoned section of the body when he runs into the Influenza-infected Cells. As he has never partaken in a battle before, he gets Scared and then he's almost killed, but White Blood Cell saves him. Then White Blood Cell fights the Influenza, but he can't make much progress because Naive's too afraid to fight. However, Macrophage arrives and assists White Blood Cell, and then she examines the Influenza, discovering it to be a Type-B virus. Then she contacts Helper T Cell, who sends the T Cell division towards their location.
Then White Blood Cell and Macrophage clear out the remaining Influenza virus. Then Naive asks the two to tell his senpais that he managed a kill, but Killer T Cell and the T Division arrive. Then they immediately know that Naive didn't contribute and start yelling at him before dragging him off to combat the Influenza epidemic. Then the other Neutrophil arrive after the T Division leaves.
In another area of the body, the influenza begin their rampage. Macrophage, the T and Neutrophil division engage the enemies. The T Division get Naive to fight, but he becomes terrified and drops his weapon, forcing Killer to save him. The division then starts yelling at him again, causing Naive to run off. Elsewhere, normal cells run away which it prompts Red Blood Cell and Senior Red Blood Cell to ask a cell what's going on, to which he informs them of an Influenza epidemic.
Then Naive runs off to the Dendritic tree where Dendritic Cell asks him what's the problem. Then Naive tells him that he's too scared to fight and that his senpais keep yelling him for not contributing to the battle. Then Dendritic Cell grabs several photo albums, telling Naive to look at the photos. Then Naive opens the albums and realizes that the T cells in the album are his senpais when they were younger. Then Dendritic explains that his senpais were also Naive T cells and that they evolved into the Killer T Cells that they are today, stating that they see a part of themselves in Naive. At the battlefield, Killer T Cell and a few other members of the T Division are in mental pain, stating that "someone has peeked into their pasts". As a result of now knowing his senpais' upbringing, Naive transforms into Effector T cell.
The Influenza virus continues to multiply and infect more normal cells to bolster its ranks. Then the group's overwhelmed by their sheer numbers. As an Influenza tries to charge at the group, a fist breaks through the wall and sends it flying. Then Effector T cell introduces himself as the former Naive and reveals that he has also proliferated, allowing them to battle the Influenza. B Cell also arrives with his antibodies, but he's ignored by everyone as they're complimenting Effector, which it annoys him.
After a week of battling, the Influenza's all, but eradicated and the group celebrates their victory. Then a single Influenza appears to confront the group; as Effector charges at it, the Influenza quickly slaps him aside and then he reverts back to Naive. Then B Cell tries to spray his antibodies on it to no avail, just as the group realizes that it's a Type-A Influenza. B Cell runs off to make Type-A antibodies as the Influenza multiplies again, forcing the White Blood Cells into another fight.