part story "The Ambassadors of Death" was unique in that it was an  experiment in changing the format of the opening credits that was not  repeated. At the start of each chapter, the credits would begin as usual  but end right after the title "Doctor Who" appeared (before the episode and  writer titles appeared). A brief teaser then followed, followed by the  remainder of the opening sequence, as usual. Another story from the same  season, "Inferno", also altered the format of the opening credits slightly  by showing the episode and writer titles over footage of bubbling lava.