Pre-emptive TL;DR: I over-analyze the crap out of connections between the first and last arcs of the first season.
So, I first watched Higurashi (including Kai and Rei) a good 6-7 years ago, and now I just finished re-watching the first season, and I had some revelations I wanted to share about the connections between Onikakushi-hen (eps 1-4) and its answer arc Tsumihoroboshi-hen (eps 22-26), and maybe see if anyone can help fill in the gaps. It is worth noting I haven't re-watched Kai yet, so I don't remember it very well right now, some of the gaps might be covered there, but I think some of it is more open to speculation.
Obviously, MASSIVE spoilers ahead.
So, during Episode 25, Keiichi "remembers" beating Rena and Mion to death at the end of episode 4, during a different version of the timeline. One thing that is important here is that, just before this, Mion makes the distinction between keeping secrets from friends (the idea Rena has been obsessed with just prior to this) and having things that you simply don't want to remember, let alone discuss. The first part of these cross-timeline memories we see is when Keiichi had just learned about Satoshi's disappearance and disavowed his friendship with Mion and the others because "friends don't keep secrets," the very same ideology being used by Rena.
But it is what the next set of cross-timeline flashbacks reveals that really turns the entire Onikakushi-hen on it's head: we are shown that the syringe Mion had in episode 4 was actually a marker, which he hallucinated to be a syringe! We then see Rena holding out her arms, pleading with Keiichi "Believe me" before he swings that bat down on her head. Based on everything else we learn throughout the season, as well as some of the details I do remember from Kai, and the simple fact that Keiichi then clawed his own throat out, we can infer with almost absolute certainty that in Onikakushi-hen, Keiichi was taken over by the same virus infecting Rena in Tsumihoroboshi-hen, known for turning its victims violently psychotic. Going back and re-re-watching Onikakushi-hen with this idea in mind, the story of those first four episodes changes drastically.
Onikakushi-hen follows the story through Keiichi's point of view, where Rena and Mion seem like the psychotic ones. But with these new revelations, we realize it's actually Keiichi who is losing his mind, everything that made this arc so terrifyingly gripping is literally all in his head. Or rather, it may be more accurate to say that it's all misinterpreted by his mind due to the effects of the virus, and we as viewers are made to misinterpret it all as well because we are watching it through his eyes. Some examples, explained through Rena and Mion's viewpoints:
The most obvious example, of course, is the one that they flat-out told us in Tsumihoroboshi-hen (with a little of my own narrative flare added, admittedly): Rena and Mion try to brighten Keiichi's spirits and return things to normal by playing the punishment game for failing his sweet rice ball homework. Rena grabs him and holds him as Mion pulls out the marker (it is worth noting that this is the exact same punishment game as the one from the game of Old Maid in episode 1, right down to Rena holding him while Mion used the marker! This explains Mion's response to Keiichi asking what the "syringe" is, when she says "you know what this is.") However (in moments we never actually see), Keiichi is panicking way too much for this, and quickly turns violent, forcing himself free from Rena's hold, and before they can react, he has pulled Satoshi's bat from its hiding place and cracks Mion across the head with it, swinging it down on her lifeless body a few more times, splashing Rena with Mion's blood (she is bloody while still alive in episode 25's flashback). The terrified girl pleads with him, trying to apologize (Keiichi mentions "words of redemption" while thinking about the flashback) and explain that they were just playing around, begging him to believe her even as the bat swings down on her skull.
Speaking of the sweet rice balls, the needle may be the next easiest to explain if you pay careful attention. Of course, one could easily hand-wave it as a hallucination like the syringe, but there is actually another, less hand-wave-y solution: Rena says that the rice balls were made by Mion's grandmother, so it is not difficult to assume that neither Rena nor Mion had any knowledge of the needle, and it likely ended up in the rice ball by complete accident, and Rena's comment about having put something in one of them herself is simply a red-herring intended to cast the blame on them. That may still sound hand-wave-y to some, but it is further substantiated through subtle narrative cues during instances when the incident is brought up later: When Keiichi tells Ooishi about the needle, he straight-out admits that it may have been a coincidence, but this is ignored due to their collective bias that Rena and Mion are after him; and further, Keiichi references the needle later when Mion is scolding him about his batting practice, but he never actually says that it was a needle, creating a very easy path to misunderstanding.
If you watch the conversations in which Rena and Mion seem to suddenly turn psychotic and their eyes become evil-looking slits (an effect that, if I'm not mistaken, was only ever used in Onikakushi-hen and the opening theme; other arcs used the more typical dull, lifeless-looking eyes instead), but keeping in mind the idea of only Keiichi being mental, some of these conversations are actually very easy to see as being normal conversations that are only creepy because of Keiichi's mindset and the atmosphere created in the show. Perhaps the best example is the one near the end of episode 3, when Keiichi skips club activities and Rena follows him. Keiichi hears her words about how she doesn't want him to transfer as a threat, and warps her tone in his mind (and thus for the viewers as well), when in fact it is easy to imagine those same words as a desperate plea, begging him not to put everyone through the the same pain they felt when Satoshi disappeared. Even some of the ones when Rena is clearly yelling in anger can be given some context when you consider that she would feel betrayed or like her friend is keeping secrets and lying to her (see he philosophy on this in Tsumihoroboshi-hen). Admittedly though, some of these moments are a bit harder to explain, particularly those near the end of episode 4, but we'll get to that...
At the end of episode 2, Rena goes to Keiichi's house, bringing tea with her, to apologize for getting upset with him during their earlier conversation about secrets (This is after I've viewed that conversation similarly to the previous bullet point). But as she reaches the door to his room, she stops, hearing him talking on the phone about the Watanagashi deaths and Satoshi's disappearance. She is offended that he would is talking to some stranger about all of this instead of his friends and keeping it a secret from them, the very thing that she got upset about in their earlier conversation. Frustrated and perhaps even depressed, she leaves without seeing him, and when Keiichi learns she had been there and heard him, he interprets it as targeting him.
Perhaps the most disturbing of these revelations aside from the truth of Mion and Rena's murder, is the beginning of episode 4 when Keiichi slams Rena's hand in the door violently, yelling at her to go away as if possessed. During this moment, between her cries of pain and please for him to stop, Rena mentions that she was joking, implying that perhaps her pulling at the chain may have been, in reality, more playful than what Keiichi saw in his mind, or could even allude back to her knowledge that he was having cup ramen from the convenience store: perhaps this could have just been a joke based on past instances of him doing exactly that, and happened to be right (other possible explanations exist too: coincidentally seeing him buy it at the store, or whoever presumably told Rena he was home alone had seen him buy it). But the extent of how traumatizing this scene really was in reality comes when she is standing outside his window in the rain: for one thing, looking closely, you can see that her hand is bleeding, and fairly significantly; and this thought process explains exactly why Rena was standing there (apparently for some time, as hinted later in the episode) repeating an apology over and over: her friend just effectively attacked her while telling her to go away as if he were possessed by a demon (yes, that is an intentional reference to the Onigafuchi origin story from other arcs), having suddenly become angry and violent toward her recently, and she has no idea what she's done wrong.
Seriously, I highly recommend re-watching that scene from episode 25, then all of the first 4 episodes keeping this change of perspective in mind. It's fascinating to see just how much of the arc can be explained and have its meaning changed just with this one simple revelation. Some of it really isn't much of a stretch of the imagination at all once you're looking for it, though there are a few things that are a little tougher to explain...
In particular, some of the creepy conversations with the evil-slit-eyes are a kinda tough, especially near the end of the arc... In episode 3, Rena and Mion somehow know about Keiichi having a heated discussion with Ooishi at Angel-Mort even though they should have been at school. The best I can think of is that Shion saw him there, since she works there, and recognized him by Mion's description (or Mion recognized him by Shion's description) and told her sister about it, but I have no actual evidence to support this beyond simply that it's a possibility. My ideas about Rena knowing Keiichi's dinner at the start of episode 4, mentioned above, are also similarly unsubstantiated, though possible. Later, when Rena confronts Keiichi on the path from school with her "hatchet" in hand, this one definitely feels off even keeping all this in mind. Granted, her strong belief in the Shrine God and its curse is well established in other parts of the series, including Tsumihoroboshi-hen, and the "hatchet" could be explained by her going to work on digging out the Kenta-kun/Col Sanders doll, but the way it's used and the general tone of the conversation still make it difficult to imagine this as being a normal, not creepy conversation even if Keiichi wasn't going crazy. Finally, slightly before this, Mion grows extremely angry and says that she should have "killed that old man back then" when Keiichi refers to Ooishi, and says she showed mercy because he was supposed to retire this year. That's hard to imagine as normal, but it is worth noting that Keiichi doesn't actually use Ooishi's name here until after these comments, and Mion never does (unless it was only spoken but not written into the subs I read, and I missed it).
Another interesting question, which admittedly might have been answered in Kai and I don't remember it, was the note Keiichi left behind his clock. Specifically, that the marker which he thought was a syringe had been removed, as well as the fact that the section removed from his note was the section referencing the syringe. Also, the second part of the note asking them to find the truth of the whole thing was not something we ever got to see Keiichi write, which with the marker and section of the note removed, begs the question of whether it was really him who wrote it. Personally, I think it sounds like something Rika would have written, and that removing the confusing false-evidence of the marker referred to as a syringe would have served her purpose, but it's hard to say unless they brought it up in Kai, especially since we don't know when she could possibly have had the chance.
And, even without looking at this idea of Keiichi being infected with the virus, there are a couple of interesting questions about exactly how much Onikakushi-hen and Tsumihoroboshi-hen connect. Remember, in the latter Rena murders Rina and Satoko's uncle, chops them up and temporarily hides them in the garbage dump until her friends find out and help her find a new place to hide it. Re-watching episode 1, Keiichi's joke to Tomitake that maybe Rena is checking on a body she dismembered suddenly stands out, and we have to wonder if this is a bit of narrative irony planted for re-watchers, hinting that these murders also occurred prior to episode 1 in the timeline we saw as Onikakushi-hen, only in that timeline Keiichi was never involved. Further evidence for this idea, though vague and possibly unrelated, comes in episode 2, at the very end of Keiichi and Rena's conversation about secrets, when she whispers to him "we also have something to hide," which could be implying that she had carried out the same murders in this timeline and received help from the rest of the group, but Keiichi was not involved. It's also worth noting that the Kenta-kun/Col Sanders doll that Rena wanted from the dump in Onikakushi-hen was also seen in Tsumihoroboshi-hen, in with the pile of furniture that her father was throwing out at Rina's request. This may be a connection between the two: they had owned it prior to Rina showing up, and it was thrown out, but Rena wanted to get it back during this timeline.
And one more relatively small thing that I noticed while re-watching Onikakushi-hen again: It's said that the Shinto priest, who we find out was Rika's father, died suddenly of a "strange illness," and his wife (Rika's mother) supposedly committed suicide by drowning herself in a lake, but her body was never found. Is it possible that the priest contracted the very same virus as Keiichi in Onikakushi-hen, Rena in Tsumihoroboshi-hen, and presumably Tomitake in basically every arc (at least, if that really is what it was for him, can't remember the answer to that from Kai right now), drowned his wife in the lake during a fit of psychotic virus-induced rage, then clawed his own throat out?
Well, that's all I've got right now. I'd love to hear some other fans' thoughts on all of this. I'm sure I'm not the only one in the 10 years since the show came out to have come up with all of this, but I didn't see a similar thread as far as I could tell, so I'm interested in what other people have to say. If you made it this far, thanks for putting up with me!
You are supposed to notice those things upon re-watching it. But a lot of the things you mentioned are explained in better detail in the visual novel and the manga arcs.
Keiichi is infected, as you said. Mion and Rena are not because like you said he's hallucinating. Also Rena and Mion tell him they are giving him "the same treatment as Tomitake" when they're about to use to marker on him( referring to the time at the festival. Tomitake played the club activity with them and he lost. As the punishment game they used the marker on his shirt ). Since Tomitake just died recently, Keiichi misinterprets what they said and thinks that they mean they are going to murder him in the same way that Tomitake was killed.
This isn't really explained in the anime but the "needle" in the food wasn't actually there at all. They put hot sauce in the food and it was so hot that it made his tongue bleed and Keiichi hallucinated the needle, which is why when he was looking for it in the trash later he couldn't find it.
And Rena knew what food he was eating she ran into his mom while she was grocery shopping.
Rika's father was poisoned and her mother's brain was dissected by Takano. I don't know if they went into detail on that in the anime or not. But it's explained what happened in the manga.
The manga has alot of details in it that the anime left out. But it's alot of more graphic. And there are certain scenes I'm personally glad were'nt animated.
Interesting... I hadn't anticipated there being quite so much difference between media, though when you say it, it makes perfect sense for a series like this. I may have to look into the manga and see for myself just how much there really is to this story.
