I almost forgot -- here are a few bits of trivia and references to go with the review of "Full Circle"
* The Doctor and Romana recieved the summons to return to Gallifrey at the end of the previous story "Meglos"
* In talking about returning to Gallifrey the Doctor mentions seeing how Leela and Andred are doing. Leela was the Doctor's previous companion before Romana and she left the Doctor to move to Gallifrey and marry the Chancery Guard Andred during "The Invasion of Time".
* Romana is upset about being recalled to Gallifrey and the Doctor responds that she had only come to help him with the Key to Time. This refers to the fact that the powerful White Guardian tasked the Doctor with a quest to find the Key to Time. To aid him in this quest the White Guardian impersonated the President of Gallifrey to convince Romana to take on the task in "The Ribos Operation". At that time Romana was in her first incarnation. At some point during the quest she learned that she had not really been assigned to job by the President but by the White Guardian but she stayed anyway. In fact, once the quest had ended she remained with the Doctor. Presumably it has either taken the High Council of Gallifrey this long to figure out where she was or else they didn't care until now.
* Romana, not wanting to give up traveling with the Doctor, reminds him that he fought the Time Lords once before; "And lost" he replies. This is likely a reference to the fact that the Doctor was originally a renegade but was caught by the Time Lords while in his second incarnation. After being convicted of meddling in time -- a crime by the standards of the Time Lords -- the Doctor was forced to regenerate and was exiled to Earth. The conviction happened at the end of "The War Games" and the Doctor began his exile with "Spearhead from Space".