The episode was the first to be set in modern Cardiff, and established that the Cardiff Space-Time Rift, implied in The Unquiet Dead, was still present in the 21st century and releasing enough energy to fuel the TARDIS. It thus laid critical narrative groundwork for Torchwood's central theme. It also introduced the extrapolator and established the energy present at the heart of the TARDIS — both crucial elements of the series 1 finale. Finally, it contained the main characters' first major recognition of the Bad Wolf meme.