in 1967 when he directed the Patrick Troughton serial The Enemy of the World. This was a complex serial to direct as Troughton played both the Doctor and the dictator "Salamander" in the same story and sometimes in the same scenes – a rare and demanding directorial requirement for the 1960s. However, in his memoir Who and Me, Letts related how he naively used matte boxes to allow Troughton to act face to face with himself, when in fact optical printing was already available and the same could have been accomplished in post-production. He became the series' producer in October 1969, replacing Derrick Sherwin, with Jon Pertwee recently cast as the Doctor. Letts' first story as producer was Pertwee's second, Doctor Who and the Silurians, and he remained the producer for the rest of the Pertwee serials, becoming the father figure in the 'family' atmosphere that had developed on the show at that time. It was an era of substantial change for Doctor Who, with episodes broadcast in colour for the first time and an improved budget which enabled more location filming and action sequences than had previously been possible. Letts also embraced the technological innovations which came with moving the series into colour, most notably his enthusiasm for Colour Separation Overlay. He also oversaw the celebration of the programme's tenth anniversary in 1973, uniting the first three Doctors in the first multiple Doctor story, The Three Doctors. When he took over, a recent BBC decision had cut the season length from over 40 episodes a year to 26. Notable changes Letts made, as related in his autobiography Who and Me, included producing the show in two-episode blocks, rather than as separate episodes: rehearsing two episodes for a fortnight, and then recording those two episodes back-to-back, thereby reducing the demands on the studio scenic crews, who only had to erect and strike the sets once a fortnight instead of once a week. This was a profoundly significant change: it allowed much more rehearsal time, in a much less frantic atmosphere; it ended a long running dispute with the unions representing the technical crews; it reduced wear-and-tear on the sets (and the budget allocations for repairing the damage); and it meant that, forever after, serials could only be made in multiples of 2 episodes—a primary reason for the 4-episode and 6-episode format dominating the schedules for the following fifteen years. He