The Parting of the Ways
Most companions travel in the TARDIS with the Doctor for more than one adventure. Sometimes a guest character takes a role in the story similar to that of a companion, such as photographer Isobel Watkins, who plays a significant role in "The Invasion" (1968), or Lynda in "Bad Wolf" and "The Parting of the Ways" (2005). In the revived era, some guest characters have gained companion status such as Mickey Smith, River Song, Wilfred Mott, and Craig Owens.