OK–undetectable Cylons infiltrate/undetectable Skrulls infiltrate.
Nobody knows who to trust.
Is Secret Invasion a Battlestar Galactica rip-off?
Seriously?
This is one of the oldest ones in the book - Invasion of the Bodysnatchers, anyone? Or any other ‘commies among us!’ paranoid fiction from the 50s.
The only aspect they have in common that hasn’t been done to death is the religious aspect. But in the current world climate, that’s pretty much inevitable. (And ripe for subversion… MI-13 makes a point of giving the finger to the real-world issues that made it inevitable with the character of Faiza - a Muslim and one of the most patriotic characters in the series.)