This season saw the introduction of the Black and White Guardians as well as the companion Romana I who became the first Time Lady companion since Susan. This season is sometimes colloquially known as The Key to Time season after the story arc involving the Key to Time. Although not the first season to consist of linked stories (Season 8 in 1971 was the first, as all the stories that year featured the Master), it was the first to be recognised as having a series-long arc. Although multi-serial arcs would occur again, such a full-season experiment wouldn't be attempted again until Season 23. The arc has a number of similarities to the Season 1 story The Keys of Marinus. The storyline was originally proposed for Season 15, but it required more planning.