Energy is the thread physicist Jim Al-Khalili pulls on to connect engines, stars, and living bodies in this half of his two-part series on order and disorder. He traces the concept from its industrial-age origins, when engineers first tried to work out why steam engines lose efficiency, through to the law of conservation of energy: the rule that energy can change form but never vanish or appear from nothing. Al-Khalili sets that law against the second law of thermodynamics, which governs how energy degrades and disperses even as its total amount stays fixed, and uses that tension to explain why perpetual motion is impossible and why every process, from a car engine to a star burning hydrogen, has a direction it cannot reverse. He moves between historical demonstrations and modern examples, showing the same conservation principle at work in orbiting planets and in the metabolism of animals. The film's case is that a single accounting rule for energy underlies systems that look unrelated on the surface.