Plim Plom Plim

It starts because Mizuki’s like “it would be really funny if I made Rui think I’m trying to kill him,” so puts a tiny amount of almond extract in something on impulse, expecting him to notice, say something about it, leading into an opportunity to gaslight him, for fun. A classic one-off bit. Rui pauses fractionally when bringing the food to his mouth, implying that he might smell the almondiness, but then he eats it anyway without comment. Which is insane, because Mizuki expected him to say something, who the hell wouldn’t notice an unexpected almondy taste? Now, more than ever, she really really wants to get a reaction out of him, so she just keeps “poisoning” him. How far can she push this?

Rui, for his part, absolutely immediately notices the almond extract, thinks there’s a very low but possible chance it’s cyanide, and thinks she absolutely wants him to think it’s cyanide. In that instant, he knows she wants to see his reaction, and will not give her the satisfaction, for fun, even if it means maybe ingesting a bit of poison. He figures that if she really wanted to kill him, he’d already be dead, so he may as well see where this goes.

For what it’s worth, Mizuki wouldn’t completely ruin every meal with almond extract, just a single drop on a small piece that she makes really obviously suspicious and easy to avoid if Rui wanted to. Like putting it off to the side, or having a fork already stuck in that piece when giving Rui his plate. And she’s definitely not cooking for him every day, I think they take care of most of their meals separately and alternate making dinner / buying food between the two of them. To meet her quota of “poisoning” him once per day, on the days she doesn’t cook, she makes tea for herself and asks if he wants a cup while she’s already boiling the water. He says sure, and Mizuki of course always puts a drop of almond extract in his. Rui notices the first time and then can reasonably expect to be poisoned every time she asks if he wants tea, and he still always drinks the tea, because he’s committed to playing along, and also it actually tastes pretty nice, cyanide or not. He’s like 94% certain it’s almond extract.

And because of the difference in their eating habits, Rui not eating vegetables, Mizuki waiting for things to cool down, it would already be the case that their plates can’t be switched and Mizuki would more often than not watch Rui eat first, which is a normal routine for them but really adds to the suspicious ambiance. The cherry pit on top, you could say. It’s incredibly blatant that Mizuki is tampering with what she serves him, and Rui never once says anything about it, but tries to throw her off in other ways.

Honestly, given his bullshit OP genius shtick, it’s possible that Rui just straight up has some actual cyanide for bullshit science purposes and, crucially, hydroxocobalamin, an antidote to cyanide. Hydroxocobalamin is also a form of B12, and though Rui has the exact opposite dietary restriction from what might result in a B12 deficiency, if he takes any other vitamin supplements to replace vegetables, it would be easy to lump that in with them, have it just in case of the off-chance that Mizuki is actually poisoning him, and offhandedly mention to her that it just so happens to be an antidote to cyanide, in order to make her think he thinks she’s actually poisoning him without ever saying that he suspects her. And Mizuki is then left with the implication that Rui might think he’s actually being poisoned but is willingly ingesting the poison she serves while taking an antidote, for some fucking reason.

They engage in further psychological warfare, of course. Mizuki changes her perfume to something with notes of almond, Rui notices and compliments her on it. Rui buys almond pastries to share, saying he’s been peculiarly fond of almonds as of late. Mizuki buys cherries and apples and other such fruit with notable cyanide content in their seeds. Rui looks up the lethal dose of cyanide where she can see his screen, and quickly closes the tab when he “notices” her catching a glance. Mizuki gets almond blossom branches to put in a vase. Neither of them ever say anything about the meals or tea.

Mizuki secures her victory by forfeiting. It goes like this: she runs out of almond extract. Using like 1ml per day, presuming a somewhat used typical 43ml bottle, there’s reasonably about a month of “poisoning” Rui. Then she runs out, and instead of running to the store to pick some up and continue the game, she simply asks if he wants tea, as routine. He says sure, and he immediately notices the absence of almond extract. Surprised, he slips up, and says it tastes different than usual. That’s cause they just ran out of almond extract, she says. He must’ve grown used to the taste cause she’s been putting it in all his food the last month, she says. He’s entirely thrown by her giving up the game, just straight up admitting that she has been tampering with his food, and that it has in fact just been almond extract. Why, he asks. Why try to make him think he was being poisoned at all?

Well, Mizuki figured he’d be so preoccupied with the threat of cyanide, he wouldn’t notice the [whatever’s funniest to be dosing him with].