Naive T Cell is 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 becomes terrified and is almost killed when U-1146 saves him. U-1146 fights the influenza, but cannot make much progress because Naive is too afraid to fight. However, Macrophage arrives and assists U-1146, and examines the influenza, discovering it to be a Type-B. She contacts Helper T Cell, who sends the T Cell division towards their location.
U-1146 and Macrophage clear out the remaining influenza virus. Naive asks the two to tell his senpais that he managed a kill, but Killer T Cell and the T Division arrive. They immediately know that Naive did not contribute and start yelling at him before dragging him off to combat the influenza epidemic. 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 prompt AE3803 and AA5100 to ask a cell what is going on, to which he informs them of an influenza epidemic.
Naive runs off to the Dendritic tree where Dendritic Cell asks him what is the problem. Naive tells him that he is too scared to fight and that his senpais keep yelling him for not contributing to the battle. Dendritic cell grabs several albums, telling Naive to look at the photos. Naive opens the albums and realizes that the T cells in the album are his senpais when they were younger. 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 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. The group is overwhelmed by their sheer numbers. An influenza tries to charge at the group, but a fist breaks through the wall and sends it flying. 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 is ignored by everyone as they are complimenting Effector, which annoys him.
After a week of battling, the influenza is all but eradicated and the group celebrates their victory. A single influenza then appears to confront the group; Effector charges at it, but the influenza quickly slaps him aside and he reverts back to Naive. B Cell tries to spray his antibodies on it to no avail, just as the group realizes that it is 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.