Brian Greene walks through one of the stranger implications of modern cosmology: that our universe might be one of countless others. The starting point is inflation theory, the idea that the infant universe expanded faster than light in a fraction of a second, with some patches expanding faster than their neighbors and pulling away to become separate bubbles of space-time. Greene lays out several versions of the multiverse this could produce, from bubbles nearly identical to our own, with the same physical laws and slightly different arrangements of matter, to bubbles governed by entirely different physics where nothing familiar would apply. String theory enters as one candidate explanation for how varying physical laws could arise from bubble to bubble. Other physicists appear alongside him to weigh the evidence and, more pointedly, the problem of testing a theory about universes that may be permanently outside observation. The film stays with that tension: an idea taken seriously by working cosmologists that may never be verifiable by any experiment.