Bai Zhao is a woman, a miserable corporate wage slave.
Not only does she work overtime until midnight, she also gets hit by a car and dies. Looking at herself lying on the ground, she can tell she's thoroughly dead. She even still has her wedding ring on.
She had just been proposed to, and now she's dead...
Suddenly, someone comes to collect her. "You weren't supposed to die. I can send you back. Come with me."
He says he isn't from the Underworld, nor is he Black Impermanence. He's from the Temporal Bureau.
Though she has no idea what he's talking about, she follows him anyway.
Gradually, she stops wanting to go back and does everything she can to stay at the bureau.
She wants to become an enforcer like him, immortal and able to travel through the cracks in time and space... The missions are fresh and exciting.
As for her fiance...
She doesn't need him anymore.