AE3803 and AA5100 are walking around when AE3803's shoelace gets untied. A5100 believes it to the first of a bad omen; then, AE3803 walks under a ladder, crashes into a worker carrying plates, and finally her cowlick gets agitated, causing the next three steps of the omen. AE3803 wonders what is going on, as she is getting a bad vibe, before her cowlick stops moving around.
In the apartment complex, Natural Killer Cell confronts the normal cell, knowing that he is a developed cancer cell due to his innocent facade and high development. The cell warps into a Cancer Cell, who admits that he could have finished the job easier if all three immune cells were in the same place, believing that she sent them off. Natural Killer states that they didn't leave yet; they were just in her way, along with the fact that they couldn't see through his disguise.
In another part of the apartment, U-1146 and Killer T Cell are patrolling when U-1146 is still suspicious. He suggests to meet up with Natural Killer and the normal cell, but Killer T refuses to. U-1146 stumbles onto a piece of paper and picks it up. Killer T calls U-1146 over, and the two find a cancer cell being incubated; they soon realize this is not a viral infection, but cancer cells multiplying at an unchecked rate. The two are then ambushed by a large group of cancer cells. As they fight, Natural Killer crashes down from above and yells at them for not noticing the cancer cell in their midst. Cancer Cell catches up and confronts the three, and U-1146 realizes that he had put up a show with one of the cancer cells to lure them to the complex.
Cancer Cell tells them that he wanted them to feel how it was like to be ganged up and violently murdered, before sending a mass of cancer cells at them. Killer T realizes that he had taken over the whole building long before they arrived, and the building collapses on itself. Natural Killer tells them that she didn't want to involve U-1146 and Killer T in the mission as they weren't strong enough. All three are defeated as Cancer Cell is victorious.
In another area, AE3803 and A5100 encounter Platelet and a member of her division. A red blood cell bumps into her and apologizes before running off. A5100 notices that he is carrying a load of nutrients, and the two decide to help him out. They soon encounter more red blood cells delivering an astonishing amount of nutrients to another apartment complex. They ask around and learn that a massive order was placed despite the strange request. Upon investigating, they find several cancer cells eating the nutrients, and the three run off. Despite the presence of the cancer cells, they continue to deliver nutrients to the complex.
The three immune cells battle Cancer Cell but are vastly outmatched. Killer T charges, calling Cancer Cell a "goddamn bug". Cancer Cell remembers his youth, when he and his friend were created as cancer cells due to their creator making an improper cell division. They attempt to flee, but his friend trips and is killed by the pursuing Killer T cells, who call him a "goddamn bug". Enraged by such a thought, he overwhelms the three again. Elsewhere, three Macrophage are cooking when one of them notices AE3803 and asks her why she is in a rush. She tells Macrophage that they are getting a lot of orders from one of the complexes, but something is wrong, which gets her attention. Cancer Cell begins his plan to spread the cancer cells all over the body, even though U-1146 warns him that the world cannot be sustained by cancer cells. Cancer Cell simply states that it is his duty to die, mutates a pair of wings and prepares to leave, but a cleaver flies in from nowhere and clips one of them.
The rest of the cells arrive, having been notified by Macrophage and battle the cancer cells. The immune cells battle the cancer cells, while the red blood cells spectate. In the ensuing battle, Killer T ends up getting in front of B Cell and sprayed with antibodies. Natural Killer laughs at how easily she fooled him, which also doubled as revitalizing her and easily incapacitates Cancer Cell, telling U-1146 to finish him off. As the other cells celebrate their victory, U-1146 walks up to Cancer Cell and allows him to speak his final words. Cancer Cell admits that he enjoyed being rescued and laments on how wanted to live with the other cells, but could not due to an improper cell division. U-1146 states that they defy the rules of the body and they cannot repair the defects, so they have to kill them as a result. Cancer Cell allows U-1146 to kill him, but eerily states that their battle is not over yet.
AE3803 runs to U-1146, who is quickly disinfected by the Monocytes. She looks at Cancer Cell's corpse and comments on how scary it is, but U-1146 states that he is dead. He then thanks her for calling the other immune cells to the battlefield. U-1146 heeds Cancer Cell's threat, knowing that he may have to face him again in the future.