Brian Greene hosts this first installment of the three-part series, opening with Albert Einstein's decades-long search for a single theory that could unite all of physics under one framework. Archival photographs and reenactments trace Einstein's early breakthroughs, special and general relativity, and the mathematics that bent space and time around massive objects like the sun. Greene then lays out the wall Einstein hit before he died: general relativity governs the large scale of galaxies and gravity, quantum mechanics governs the erratic behavior of particles, and the two sets of equations contradict each other whenever they're forced together, as they are inside a black hole. Animated sequences and interviews with working physicists walk through why this contradiction matters and how string theory, which proposes that every particle is really a vibrating one-dimensional string, emerged as a candidate for the unification Einstein wanted. The episode ends with the theory still unproven, setting up the deeper mathematics covered in the next two parts.