chronology and canonicity

Veteran actor Tom Baker—we learn via Slashdot—will reprise his the role he’s best known for as the avatar of the fourth Doctor to complete a mini-series authored by Douglas Adams.
The Shada arc of narrative, meant to be the concluding episodes of the long-running programme’s seventeen season, had to be abandoned back in 1979 due to a writers’ strike and never revisited for fear that production time would cut into holiday scheduling commitments. Missing scenes have been filled with animated vignettes, voiced by Baker and his companion Time Lady Romana, Lalla Ward, and revolves around a mysterious prison planet and its inmates that the Gallifreyans built to house the Cosmos’ archest of villains.