Frank is a by-the-book zookeeper, but he's not exactly a healthy guy. He knows it, his young daughter Shane knows it, and so does her teacher, Mrs. Boyd. One day, his gluttony leads him to mistakenly eat an apple contaminated by monkey saliva. The camera follows the apple down his throat and into his stomach... and thus, inside Frank's body—more precisely, in his bloodstream—we discover a white blood cell named Oz. Like all white blood cells, Oz's job is to patrol the bloodstream daily, dealing with any bacteria or viruses that dare to disturb the human body. Despite this crucial task, Oz is no rule-follower. No, he's not your typical, by-the-book white blood cell. He's naturally free-spirited and undisciplined, resisting rules and regulations at every turn, always eager to prove he's a "unique" cell with personality. Frank Detorie's condition worsens, and even his good friend Bob notices something is off. Many who see Frank feel he's no longer the strong poster-hanger he once was. Perhaps to rein him in, Oz's boss assigns him a new partner: Drix, a by-the-book, newfangled cold pill. Forced together, these two with clashing temperaments and habits inevitably bicker and argue. Just as they're at each other's throats, the apple Frank ate introduces a dangerous virus named Thrax. This powerful virus runs rampant and unchecked within Frank's body. Unfortunately, he and his partner Drix—a 12-hour pain-relief cold capsule—can only ease Frank's pain for a short while. To fulfill their sacred duty as a white blood cell and to save Frank's life, Oz and Drix decide to set aside their differences and join forces against the common enemy. They race against the biological clock: dodging mucus sludge, purging toxins, and tackling all sorts of unimaginable tasks. It's dirty work, but these antibodies have to do it—because every body needs a hero. Meanwhile, Frank visits the doctor due to his illness, and his physician is struggling to figure out just what is causing Frank's symptoms.