There's a device of some sort implanted in the neck of the MC. (It's purpose isn't 100% clear to me, since I can't read Chinese, but it seems to make her more aggressive or something? IDK)
The experiment involves changing people's personalities. The company chose people with a certain positive personality type, and unfortunately MC qualified. The test subjects were assigned to have their personalities changed and become the opposite version of themselves. For MC, she was to change from an upholder of justice into a psychopath, and Xiao Man was assigned to work on changing MC. I forgot the exact reason why, but halfway through Xiao Man did not want to continue changing MC. However, she was having difficulties undoing what she already did.
When all seemed hopeless, she found out about MC's masochism which, when satisfied, worked in curbing and sealing the psychopathic side. The transmitter was used to link Xiao Man to MC, because they aren't keeping MC locked up in the company for those experiments lol. Xiao Man used that connection to continue slapping MC's ass, until the transmitter stopped working a year before their meeting. Despite the transmitter malfunctioning though, MC continued to dream of Xiao Man thanks to her own subconscious. It was from her imagination this time.
The dark MC readers here would see merging back into MC? That's the artificial psychopathic character who broke loose, and you can tell that MC was immediately struggling to control the negative impulsions. The rape? MC lost control, that's it. She's aware of it all, but all too pissed to care and took it too far (like a psychopath). The MC isn't too happy after the fact because she herself isn't a complete psychopath; Rather, she just wanted vengeance for what seemed like Xiao Man satisfying herself by approaching and using MC. Vengeance isn't satisfying of course, not when MC was a good person at heart. Conveniently she regained her senses after she got beaten up and felt physical pain.
The old man is Xiao Man's teacher, who revealed everything about what Xiao Man did to MC and how Xiao Man was actually trying to save her from becoming like the other test subjects, most of whom had committed suicide and that last one we saw going insane.
The police office investigated the transmitter MC gave, and found out that it's by the huge ass company Xiao Man is from. No records about it since that's where the sun doesn't shine. He warns MC about investigating the company.
Despite the rape, Xiao Man still wishes to save her. Obviously MC doesn't trust her completely, but she decided to trust her somewhat after doing her own investigations because she dreamt that the psychopathy would eventually consume her.
Then Xiao Man's first real life BDSM experience took place ^q^