The near-miss with the car brought back Kagari’s memories, but only some of them. She’s still missing a 12-year gap between 10 and 22. As a result, Kagari acts a lot more like a child than she used to, and treats a somewhat bemused Mayuri (who is mostly going with the flow) like her beloved “mommy.”
Watching a 22-year-old woman act so spoiled around her parents irks Suzuha, to the point they have a yelling match in the TV repair shop. Both sides regret the fight and plan to apologize, but Suzu learns something crucial from it: her and Kagari’s memories of how they became separated are very different.
After conferring with Tennouji, Rintarou begins to suspect Kagari’s strange memory gap is the result of foul play: brainwashing and mind control, just as Kiryuu discovers…something less than 5km from where Kagari collapsed. It’s a clue, but it requires they take a long drive.
Mayuri decides to celebrate the restoration of at least some of Kagari’s memories by throwing one of her patented parties, which she tries to make a surprise, but with her early memories restored Kagari knows when her Mommy is trying to keep a surprise party secret.
All the while, this ominous van drives around Akiba playing seemingly innocuous Mozart, and it’s clear the van is Bad News, whether it’s a van for kidnapping or simply for triggering Shiina Kagari. That perilous van hangs there, like Damocles’ Sword, over the remainder of the episode, as Mayuri & Co. prepare the party.
If the argument got the ball rolling on a theory about mind control, Kagari’s desire to properly apologize to Suzuha is the unfortunate side-effect. Kagari’s trip to the sweet shop isolates her from everyone else, who in hindsight are wayyy to loosy-goosy with her security at this point.
Indeed, in his desire for more clear answers about what’s going on, Rintarou is far, far away; in no position to keep her safe.
She hears the Mozart from the van (which is either planted there by “Them” to play specifically for her, or sheer coincidence) and more memories flow into her head: memories of being left with “doctors” by Mayuri, ostensibly to cure her PTSD, but the visits really comprise a kind of human experiment called the “Amadeus System”, of which Kagari is Sample #K6205.
The shock of this influx of memory sends Kagari into a trancelike state, and she drops the cake for Suzuha and her cell phone and wanders off who-knows-where, believing she’s hearing “the voice of God.” More likely, it’s the voice of those who did this to her to begin with.
Combined with Rintarou and Kiryuu discovering the facility, where Kagari was held in a cell for who knows how long, scrawling “Mommy” on the walls, Kagari’s vanishing from everyone’s sight (again) forms one hell of a thrilling cliffhanger for the second half of Steins;Gate 0.
While we may now know mostly what’s been done to Kagari, it remains to be seen who did it, why, and most important, how Kagari is linked to Maho and Leskinen’s Amadeus System. Was Kagari even a war orphan from the future? Will there really be enough cups and plates? We shall see…