Tsumihoroboshi-hen's fourth chapter, Invasion of Earth opens with Rena taking all of the cash from her father's cash box (at least 10,000 yen) and escaping to take refuge in the junkyard, as she believes that she must keep moving if she does not want to be caught by the Sonozaki henchmen. Her father later discovers the next day that the cash box was emptied out and that Rena has never return home since the day before, so he pleads Tatsuyoshi Kasai, whom he coincidentally ran into that day, to search for Rena and bring her home.
Kasai informs the Sonozaki family and other bodyguards of the situation, and they begin an active search for her. However, this information somehow leaked to the Okinomiya police station where Ōishi works, and then the police began an active search for Rena as well. Rena, oblivious to what Ōishi's policemen and the Sonozaki henchmen actually look like, automatically assumes them as threats and escapes from their sights.
At the same time, she has reached LV5 Hinamizawa Syndrome and begins itching at her neck and all over her body. She starts hallucinating apparitions of aliens with red eyes, and suddenly proclaims that her mission is to now protect Hinamizawa; the 3rd "Lunch Takeout List" TIPS from the previous chapter, written by Rena's mental doctor Dr. Kuroda, suddenly become true:
- "Delusional mood is the feeling of impending crisis without basis.
Delusional intuition is the feeling of a sense of duty or goal without basis.
Delusional perception is the feeling of a baseless cause against a baseless target."
Meanwhile, Mion visits Keiichi to tell him that Rena disappeared, and also that she actually moved the cut-up bodies of Hōjō Teppei and Mamiya Rina from the Takatsudo forest to the Sonozaki underground sewer tunnel, since there was a brochure sent that contained details of a reforestation plan taking place in the forest area (see "Notice From the Forestry Preserve"). She also tells him that she has just sent a false tip to the police station in order to widen their search range and discourage them from searching every spot in Hinamizawa.
Both of Mion's actions are misinterpreted by Rena; the false tip was that Rena was actually seen at the granary, so Rena assumed that the "aliens" created a clone of her to take her spot once she is silenced. The removal of the corpses is interpreted as Mion's betrayal against Rena, and Rena assumes that Mion plans on turning Rena in to the police. She then calls Ōishi to get a background check on everyone, and returns to her hideout.
Before Keiichi comes to visit Rena at the junkyard, Rika shows up and throws Rena a syringe of the C-120 medicine, telling her that the syringe will help her settle down. However, Rika only uses her serious voice towards Rena, prompting Rena to assume that Rika's true personality is another clone produced by the "aliens". Rika (who has already chosen to give up on this world) plays along and confirms that she indeed is not "the real Rika", and has lived in Rika's body for over a hundred years.
Once Keiichi arrives at the junkyard, Rena has already begun packing to escape to another hideout, believing that Rika will tell the aliens where she is. She explains to Keiichi about the existence of the alien parasites, and then denounces Keiichi as not a true friend, because he never told her about the toy gun scandal he committed back in the city. The next day, he confesses to Mion, Rika and Satoko about the scandal, and then suddenly has a flashback of himself murdering Mion and Rena in Onikakushi-hen.This scene is the pivotal turning point of the arc in which Keiichi finally realizes his sins. He learns an important lesson: that there are things you should tell your friends, and things that you don't have to. That being a true friend doesn't require exposing every secret about yourself and your past in front of your friends. Rika accounts Keiichi's flashback as a miracle, and has once again found the will to challenge destiny again. Keiichi, realizing that Rena's situation is very similar to his, finds the strength to go and save Rena from the terrible fate.
Later the evening, Mion, Akane and Kasai come to an agreement with Ōishi and his policemen that Takano's scrapbooks are nothing but occult stories, and that they will work together to bring Rena back home. The misunderstanding that the Sonozakis are trying to kill Rena is cleared.