Sang Tong wakes in the depths of the void, having forgotten where she came from and where she is going. She wanders through the void, and when she happens upon a creature that calls itself a System, her journey to transmigrate into a book begins.
What do you mean, those two are a couple?!
The guy with a mind full of steamy thoughts is the male lead gong? And the one who's entirely focused on being a good shizun, with no interest in romance, is the male lead shou?!
System: Blah blah... After doing this and that, the male lead gong and the male lead shou get together.
Sang Tong ponders this. "...Oh."
But in secret, she sticks close to the male lead shou, making sure that nobody with designs on him can so much as dirty a corner of his clothes.
While disguising herself with secret identities, she also gives the male lead gong a hard time, presenting him with a friendly little fitness-boosting package.
As the plotline keeps getting longer, the System falls into deep thought.
Only then does it finally realize that Sang Tong is a shou-con!
System: If the plot goes off track, the world will collapse!
Sang Tong: Don't panic. I'm here.
And so Sang Tong leisurely enjoys the increasingly interesting plot while keeping a close eye on the male lead shou, making sure the male lead gong can't take advantage of him.
Until one day, she suddenly remembers her past life and realizes her actions have a reason: she, too, was once part of the male lead gong and shou's PLAY.
Warning: The protagonist is not purely good.