Incidentally, nobody ever discovers he's a merman despite all his s*upidity. He goes through the entire story and the biggest reveal is that he somehow gets outed as a man, although it's unclear to me how this was discovered and who exactly outed him, as apparently this was revealed by a letter sent to the household. After some predictable and generic drama, the ML tries to send him away, only for the MC to disappear on his own right after curing the ML's legs.

Oh, and the ML apparently knew that the MC was the fish he saved years and years ago, and that he was a merman, because of the day of the ML's mother's death anniversary when the MC thought he was alone with the memorial tablet and told the mother he wanted to repay the ML for saving him even at the cost of his own life, and the ML happened to be outside and overheard him. Somehow, despite the MC being super vague with his words, the ML's big brain leapt to all the correct conclusions that 1. the MC was the fish he saved on the beach and threw back into the ocean years agon and 2. the MC is also a merman. So for a huge chunk of the story he apparently knew everything about the MC, which makes his behavior in retrospect even more a**holish.

His evil concubine-stepmother's evil family had evilly plotted to have the ML evilly killed so that the stepmother's evil son could then inherit the title and wealth. Only the attack failed and instead of killing him, only succeeded in paralyzing his legs. The father is reluctant to name his second son (the stepmother's son) as heir since he's a wastrel and technically secondborn and the son of a concubine and not the (now-dead) main wife, but he's also aware that the ML is physically compromised and... I'm not sure I get why it's such a big deal since he was a promising young general and like saved the country already or whatever so it's not like he's disgraced, and yeah his legs now don't work and he had to retire from the battlefield, but he apparently still holds some kind of military position where he does do work and inspect troops and stuff, and he's also still capable in bed so he could technically still produce an heir of his own.

Initially I thought the issue was that he was thought incapable of having kids due to his injury so he'd be a dead-end as family heir, but that's not the case and it seems like everyone in the family knows this too, like, it's not an issue, even his grandmother expresses hope that the MC will be preggo soon.

Anyway, the whole family, from his evil stepmother to his not-evil-but-kind-of-weird dad to his supposedly-loving grandmother actually do not give two sh*ts about the ML compared to saving face and protecting the family name, as when the stepbrother openly tries to mu*der the ML at home in a fit of rage one night only for the MC to throw himself in the way and take a knife in the back to protect the ML, the rest of the family hastily comes together to scapegoat a random servant and have him beaten to death, and then repeatedly tell the MC that it was dark that night, he must have been mistaken, brother killing brother would be a serious crime, etc. When the MC tries to talk to the ML about this, the ML just kinda shrugs and is like, "Yeah I know that they know and are protecting my brother, what can I do about it."

Through vague "investigations" involving the help of the ML's friend and vaguely-second-male-lead Shen Hanyuun and a lot of letter writing, the ML finds out that the stepmother's family was probably involved in the original paralyzing attack on him, as well as other vaguely referenced attempts at maybe assassination or otherwise trying to bring him down.

Then when the MC dramatically runs away he leaves behind, among other things, a letter detailing the stepmother's treachery in far greater detail than the ML was ever able to find out (not that the story details any details, of course, it just says there was a list of her misdeeds), though how the f*ck the MC found this stuff out is a mystery, and as the story describes nothing further, it literally seems like he just wrote a letter saying she did bad stuff, with no mention of proof or evidence or anything. But I guess there must have been since apparently this letter was enough for the ML to go to the emperor and kickstart an investigation into the crimes of the stepmother's family that ended up with them disgraced and in jail, including her and her son. Which is the stuff the ML tells the MC upon being reunited, covered in a couple of sentences.

What I've just written here is probably longer and more detailed than the actual investigation-plot stuff in the story, which I remind you is 44 chapters long.

He was pulling scales from his tail to grind into powder that he secretly added to the ML's food and drink since mermaid scales have healing/strengthening properties. Household servants apparently witnessed and caught him adding a white powder to the ML's diet multiple times but also apparently didn't do anything about it or report it to anyone until the day someone drugged the ML's tea with an aphrodisiac, which of course meant that the MC simply had to engage in their/his first time hot not-at-all-awkward wheelchair smut in order to detoxify him.

The morning after, the ML is a cold a**hole to the MC and the whole household gathers to accuse him of poisoning/drugging the young master, with multiple servants coming forward clamoring about the white powder the MC has been adding to the ML's food and drink recently, and one of the maids saying the MC was the one who sent the drugged tea to the ML. Also, gaspshock, there's a bag of mysterious white powder right there fallen open on the bedroom floor where the MC dropped it in his haste to get it on with the ML.

The MC despairs because the ML is cold to him and there are so many witnesses and the household has hated him from the beginning, and he can't tell them the truth that the powder is made from his own ground-up mer-scales! That would reveal his nature as a merman! But he's "too stubborn" to lie, so he just lowers his head and admits to adding the powder and says nothing more, instead of saying "it's a health supplement and not a poison or aphrodisiac, I can prove it, I'll take some myself right now."

So he gets reviled and slapped under house arrest and drama ensues, until a while later during another confrontation the ML finally steps in and claims... it's a health supplement and he's been aware of the MC adding it to his food, and knows he did it secretly because the MC knew he'd reject it, so it was done out of love and concern for his health.

And everyone is forced to accept him at his word. Why the f*ck didn't the MC say this right when he was accused? And even if they'd demanded to know where he got it, he's been out shopping by himself before, he could just say he bought it outside.

Eventually after "investigating", the ML announces that it was actually the jealous maid who served both him and the MC who had drugged him, and she gets punished and thrown out of the estate. There was some weird backstory about how she'd been raised in the household to become his concubine or something but that didn't happen because he got married and I guess had no interest in it? It was kind of vague and didn't make a whole lot of sense, except to provide her with motivation to hate the MC and try to drug the ML so that she could get it on with him and get pregnant and force her way into becoming a concubine.

Speaking of which, that's another reason I didn't like the ML. He knew this maid was being a d*ck to the MC and although he reprimanded her once or twice after catching her during especially rude behavior, he still had her be one of the primary servants taking care of the MC instead of giving her the boot or reassigning her elsewhere.

The MC heals the ML's legs by ripping his "mermaid pearl" out of his chest and forcing it into the ML's body (?), which was he source of all his mer-powers. Now that he's given it away, he can no longer change back into a mermaid or return to the sea, and he can no longer communicate with aquatic life, and his lifespan has been greatly shortened—he's just a normal human now. Boohoo.

Except NOT! Because he can still get pregnant! And when he does, for some reason he regains the ability to change his legs back into a tail, for a reason that is never explained! He still can't talk to the carp in the pond anymore but that's okay because now that the ML has his mermaid pearl inside him, the ML has gained the ability to talk to fish, and the fish understand human speech, so the ML just has to translate one-way for the fish to reply to the MC.

Three chapters with smut : 14, 25 and 39. There is more smut in the manhua.