1920s Toronto - Fred Banting, a former Army surgeon both physically and mentally scarred by the war, bursts into John Macleod’s office at the University of Toronto. Despite having no relevant experience, Banting is convinced he has come up with a cure for diabetes, he just needs the resources to prove it. Macleod, a middle-aged Scottish Professor of Physiology, initially brushes off the arrogant surgeon. Fred, however, refuses to leave until the older academic will hear him out. His theory of using pancreatic fluid from a healthy subject to treat a diabetic one intrigues Macleod and he gives Banting the chance to prove the treatment can work, despite his disdain for the man himself.
Macleod forces Banting to take on Charlie Best, a rich, young, medical student, who also fought in the war, and Bert Collip, a mild-mannered chemistry professor, effortlessly talented, but unworldly, and prone to misread social cues.
Through Macleod and his research, Banting meets a 12-year-old girl called Elizabeth Hughes who’s staying at the university hospital with a younger orphan, Jack Stebbings. The only treatment available to them is a gruelling starvation diet, which merely delays the inevitable. Lizzie is convinced Banting can cure her and they forge a close sibling-like friendship. The sight of these emancipated children, literally starving to death as their life slips away, is a reality check for Banting. The clock is ticking, and he’s now more determined than ever.
They have just one summer to find the cure and their experimental progress is slow. Banting only manages to obtain a small amount of pancreatic extract after months of work. Temperaments are fraying and fuses shortening as the summer heat intensifies in the claustrophobic laboratory.
Banting’s frustration increases because of the lack of progress and the lack of funding. He decides to confront Macleod and demand more money for their work, but this backfires and Macleod decides to cancel the research.
Meanwhile, at the hospital, Elizabeth and Jack are deteriorating, they are walking skeletons. Elizabeth is disciplined with her starvation diet, but Jack is convinced he will not survive.
Jack eats a stolen can of peaches, falls into a diabetic coma and dies. Macleod and his wife—who have no children of their own and have taken Jack to their heart are heartbroken.
When an invitation arrives to give a lecture on their research at Yale University, Macleod changes his mind and wants to continue. Banting is determined he should deliver it. But it's a disaster. Banting fumbles his presentation; Macleod takes over and the audience is unconvinced and mock the pair.
This signifies the complete collapse in their relationship. In the argument that follows the failed lecture, Banting punches Macleod in the face. Macleod retaliates by firing Banting on the spot.
It’s over. Banting also rejects Bert and Charlie’s efforts to get him to patch things up and he heads to the hospital to drop off his notes. But while he is there, Elizabeth, now a near skeleton, collapses.
Banting needs a quick solution and takes a huge risk, he injects the extract—insulin—on himself. His blood sugar level drops like a stone: it’s worked!
Banting rushes to Elizabeth who is in a coma, on the brink of death. He injects her with the insulin he has tested, it works, and she opens her eyes. She begins to recover.
As a result, Lizzie becomes one of the first patients in history to receive insulin as a cure for diabetes. It’s so successful that remarkably, she outlives the four scientists who saved her life.
Banting and Macleod never speak again. Not even to appear on the same platform when, in 1923, they win the Nobel Prize for Medicine.