The Doctor needs to return to Gallifrey but, because of Romana's action in the previous adventure, he is dreading the paperwork and official inquiry. Instead he convinces Adric they should stop by Earth to take some measurements of a police box before heading to Logopolis to have repairs completed to the Chameleon Circuit. Arriving on Earth the Doctor senses something wrong. A time bubble has formed, causing a TARDIS to appear within a TARDIS. The Doctor fears it's an infinite regression. If so, he and Adric will be trapped forever. Meanwhile Tegan, an Australian airline hostess, enters the Doctor's TARDIS after her car breaks down.