The Tenth Doctor was shown visiting all of the companions that had appeared in the current series prior to his regeneration. The Eleventh Doctor reveals that during that time, he not only visited the current series companions, but also all the companions that appeared in the classic series as well.
When Clyde asks the Doctor how many times he can regenerate, the Doctor replies 507. This is an in-joke as the numbers add up to 12, a Time-Lord's "official" number of allowed regenerations. At the time, both Russel T. Davies and his successor Steven Moffat were being evasive about whether they would adhere to this limit, and it was not officially resolved until Matt Smith left the role.
This story takes place during the honeymoon of the Eleventh Doctor's companions Amy and Rory, placing it between Series 5 and 6 of Doctor Who, shortly before or after the Christmas special "A Christmas Carol".
Sarah Jane reveals that she has done some research on the whereabouts of the Doctor's former companions. She reveals that Ian Chesterton and Barbara Wright were professors at Cambridge and had not aged since the 1960's, that Tegan Jovanka was in Australia fighting for Aboriginal rights, that Ben and Polly were running an orphanage in India, and that a "Dorothy something" ran a foundation called A Charitable Earth (ACE, implying Dorothy Gale "Ace" McShane). Sarah Jane also says that her former co-companion Harry Sullivan saved thousands of lives through vaccines. She speaks of Harry in the past tense, implying that he has passed on.
This two-part story marks the first time former Doctor Who producer Russell T Davis has written dialogue for the Eleventh Doctor.