John Cusack plays Michael Merriman, a young Chicago physicist who discovers the hard way the dangers of the bomb he's been working on and innocently calling the 'gadget' or device.

Cusack keeps a diary for his father and narrates the film, becoming a martyr for the cause--inadvertently.

It is the bombs themselves who are referred to in the title--they've been nicknamed Fat Man and Little Boy. This film is about the abuse of power by General Leslie Groves, head of the Manhattan Project. Paul Newman is at the helm, playing Dwight Schultz, the leader of a group of scientists in charge of developing, testing and building an atomic bomb. The scientists, including Michael, are given 19 months to make it in New Mexico.

The one big flaw the scientists failed to observe was the long-term consequences of the bombs. In stead, they are so absorbed with getting the job done that they are unable to see any of the moral implications of what they are actually doing. Interesting, overall, but not the most entertaining film Cusack has been involved in.