The Sixth Doctor: The Last Adventure

 was a full cast audio anthology released in 2015 by Big Finish Productions, and is notable for being the first time the events leading up to the Sixth Doctor's regeneration have been portrayed in performed media. It is also notable for introducing the companion Constance Clarke.

Publisher's summary
A very special story which at last provides a heroic exit for Colin Baker's much-loved Time Lord. Four hour-long episodes, connected by the presence of the Valeyard, the entity that exists between the Doctor's twelth and final incarnations.

Stories

 * The End of the Line
 * The Red House
 * Stage Fright
 * The Brink of Death