Shiriri is a mysterious alien whose spacecraft slams into the Nohara family home, triggering the narrative's central conflict. Emerging from the wreckage, Shiriri unleashes an unintentional energy surge that strikes Hiroshi and Misae Nohara, causing them to physically regress 25 years into child forms, an effect enabled by Shiriri's alien physiology or technology. To evade detection and travel across Japan, Shiriri conceals itself within Shinnosuke Nohara's anatomy. Locating Shiriri's father becomes critical, as only he possesses the ability to reverse the transformation and restore Hiroshi and Misae. Concurrently, Shiriri's father advances a significant, undisclosed conspiracy, complicating the situation. Another named alien, Morudada, actively pursues Shiriri, hinting at conflict within Shiriri's species or history. The escalating consequences of Shiriri's presence and the search for their father ripple outward from the Nohara family to involve Shinnosuke's friends and eventually engulf all of Japan. The character's name holds thematic weight. The film's slogan, "That night both got ASSociated," incorporates a direct pun based on Shiriri's name, phonetically linking it to the Japanese word "shiri," referencing human anatomy and underscoring Shiriri's chosen hiding place.