It was known beforehand that Jon Pertwee would be leaving his role as the Third Doctor and that a new Fourth Doctor would need to be cast for the part. Tom Baker had previously had major parts in several films, including "Nicholas and Alexandra" (1971) and "The Vault of Horror" (1973), but had found himself unemployed as an actor and working in construction at the time. He had written to Bill Slater, the Head of Serials at the BBC, asking for work. Slater suggested Baker to "Doctor Who" producer Barry Letts, who had been looking to fill the part. Letts saw Baker's work in "The Golden Voyage of Sinbad" (1973) and hired him. Baker would continue in his role as the Doctor for seven seasons, longer than any other actor.