X-Nico

Christmas on a Rational Planet

It also contains the first casual reference to the Faction Paradox, which become an important element in the BBC books Eighth Doctor Adventures and subsequent spin-off series, and an explicit explanation that the mark carried by the Third Doctor during his exile on Earth was a branding used to distinguish criminals by the Time Lords (in reality, this mark was a tattoo of a cobra that Jon Pertwee picked up while serving in the Royal Navy).

see also