For decades the standard answer was that the universe began in a single instant of unimaginable energy, with no meaningful "before" to ask about. This BBC Horizon film follows physicists challenging that picture, starting with Neil Turok, director of the Perimeter Institute for Theoretical Physics, who argues the Big Bang was actually a collision between two parallel universes drifting in a higher-dimensional space we cannot detect. In his cyclic model, that collision is not a one-time event but the latest in an endless series of cosmic bangs and rebounds. Roger Penrose offers a rival version, conformal cyclic cosmology, in which each universe ends in a "Big Chill" as matter converts entirely to energy, seeding the next cycle's Big Bang. The film sets both ideas against their harshest critics, who object that a beginningless universe strains causality and the laws of thermodynamics, and that extra dimensions remain untestable with current instruments. What emerges is less a solved mystery than a live argument among some of cosmology's most prominent physicists.