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I just started watching season 4 of Battlestar Galactica (2004), and I'm having a hard time suspending my disbelief on this plot point. The Final Five become aware of their nature due to that song they collectively get stuck in their heads.
All Along the Watchtower, by Bob Dylan.
Why did they get this song stuck in their heads? Of all the songs possible, this song shouldn't exist in the universe which the Galactica crew inhabits.
It's also confusing that these characters would immediately recognize their Cylon nature as a result of a song getting stuck in their heads, but that's another matter and somewhat easier to explain.