In the film, the brothers watch a recording of Charles giving a remarkably prescient speech in 1970 in which he mentions the problems presented by an increasingly throwaway plastic society.
William admits that his father was affected by the backlash he received for his environmental views, which saw him ridiculed for supposedly talking to plants.
Harry confesses that he was 'programmed' to collect litter because of the positive influence of his father.
"He's just let us learn… from him, learn from Mummy, to the point where I used to get the… mickey [taken] out of me at school for just picking up rubbish," he says.