The fourth season of British science fiction television series Doctor Who began on 10 September 1966 with the First Doctor (William Hartnell) story The Smugglers and, after a change of lead actor (Patrick Troughton) part-way through the series, ended on 1 July 1967 with The Evil of the Daleks. For the first time, the entire main cast changed over the course of a single season (the only other occasion this has happened is during Season 21).