Brian Cox traces gravity from backyard apple tree to the edge of modern physics, asking why a force everyone feels remains the hardest one to explain. The film follows Newton's work in Lincolnshire before turning to Einstein, who replaced Newton's invisible pull with the idea that mass bends spacetime itself, a shift illustrated through models, archival images, and location filming that put the theory into physical terms. Cox walks through the experiments that confirmed general relativity, including light bending around the sun during a solar eclipse, and explains where the theory starts to break down, at black holes and at the quantum scale where gravity refuses to fit alongside the other forces. Interviews with physicists add context on the current search for a theory that might unite gravity with quantum mechanics. Rather than presenting gravity as solved, the film treats it as the field's oldest unfinished problem, closing on the open question of what a successor to Einstein's theory might look like.