Final answer:
The post-mortem narrator in 'Six Feet Under,' Nathaniel Fisher Sr., dies in the first episode due to a collision with a bus while driving his hearse. This incident is pivotal for the series, affecting the Fisher family and threading through the show's themes.
Step-by-step explanation:
The post-mortem narrator in the television series Six Feet Under is Nathaniel Fisher Sr., who dies in the very first episode of the show. His death is caused by a bus collision, and it happens while he is driving his new hearse. This event sets the stage for the entire series, as each episode begins with a death, which ties into the show's themes of mortality and the running of the family funeral home business. Nathaniel Sr.'s death, while not depicted in a graphic manner, reverberates throughout the series and impacts the lives of the Fisher family deeply.