Jim Allison, the brilliant, stubborn, down-to-earth and hopelessly lovable title character of “Jim Allison: Breakthrough” may not look the part of a superman. But by the end of this absorbing, gracefully constructed and deeply moving documentary, he will have audiences wanting to join the fan club and get the T-shirt.

We meet Allison as he is receiving the Nobel Prize for medicine in 2018, the long-haired, bespectacled scientist looking pleased and just a tad bewildered. “Breakthrough” then takes us to tiny Alice, Texas, where Allison grew up and had the temerity to challenge creationism in his public school.

After enrolling in the University of Texas at Austin, he began to study biology and perfect two notable talents: novel and meticulous lab research and harmonica playing. Just how this path led to the Nobel is part of the story, but it doesn’t begin to sum up what turns into a captivating homage to scientific discovery and an endearing portrait of a modest, quietly driven genius.

Directed by Bill Haney, the film reveals the profound emotional loss that motivated Allison’s work, which eventually began to concentrate on immunotherapy as a treatment for cancer.