Instruct: Given a premise, retrieve hypotheses that are entailed by the premise.
Query: The twenty-first season of British science fiction television series "Doctor Who" began on 5 January 1984 with the 5th Doctor (Peter Davison) serial "Warriors of the Deep", and ended with Colin Baker's first serial "The Twin Dilemma". For only the second time (the first being during season 4), the entire TARDIS crew changed over the course of a single season.
Season 21 began in January of the 1980's for the show Doctor Who.
The episode featured several different actors and actresses. 
At the time of the ninth episode of the eighth series of "Doctor Who," Christopher Fairbank was not a regular cast member on the show.
Christopher Fairbank has appeared on at least one episode of "Doctor Who"
Peter Davison starred in Doctor Who between 1980 and 1982.
Cerberus had more than one head.
There were 5 other people in the bank at the time.
They made changes to the show as it was airing.  
It was the sequel to the 1981 series "Arthur C. Clarke's Mysterious World"
Holding the Fort was written by two people and aired at the same time as Doctor Who.
Doctor John's decision was a very difficult one.
The UK soldiers had been captured five years earlier.
The Hound of the Baskervilles was shown after the 1950s
Mr. Buckingham's resignation was discussed.
Only one doctor was involved.
Things took a violent turn.
The Team needed a major shift.
There were many technicians working on the problem
The series stars a total of 5 people
The Guardians was a British television show
The operation did cure Dick.
The series was about a good Prime Minister of United Kingdom.
Doctor Who began its twenty-fifth season on January 5, 1984.
The Warriors of the Deep and The Twin Dilemma were both the first and last shows of the series. 
The twenty-first season the entire TARDIS crew was the only time the curse of the crew changed over a single season.