Brian Cox and Robin Ince stretch the cage of infinite proportions this week to encompass not just our own universe, but any others that might be lurking out there as well. They'll be joined by QI creator John Lloyd, the Astronomer Royal, Professor Sir Martin Rees, and solar scientist Dr Lucie Green to talk about one of the most tantalising ideas of cosmology, that of parallel universes. Are we inhabiting a universe that is just one of a possibly infinite number of others and how would we ever know? Is this an idea that is destined to remain one of the great scientific thought experiments, and a staple of science fiction, or will science ever progress enough to truly put the idea of multiverses to the test. Producer: Alexandra Feachem.