hanger of the previous story due to an attack by the Celestial  Toymaker), he would have been played by a new actor. The change of  actor would have been explained away as part of the Toymakers game.  This idea was vetoed by the BBC, but if it hadnt, its quite  possible that the series wouldnt have lasted so long as the concept  of regeneration might never have been invented. Ultimately, Hartnell  left the series four serials later due to ill health, which was when  the concept of regeneration was born.