The TARDIS materializes on Kembel where the Doctor meets Bret Vyon.  He has been searching for Marc Cory (from Mission to the Unknown).  When the Doctor discovers that the Daleks are going to use a weapon called the time destructor, they decide that they need to warn Earth.  The Doctor escapes with key elements of the time destructor causing the Daleks' plans to some to a halt.  Sara Kingdom to track down the Doctor. 
Sara is unaware that she has been sent by a traitor and kills Bret Vyon, her own brother.  Once she finds out the truth, she joins with the Doctor and crew.  They encounter the Monk they then all arrive back on Kembel.  The Doctor decided to use the time destructor against the Daleks.  Sara Kingdom is killed when she fails to return to the TARDIS.