Prior to Esther Rosenthal being invited to supervise Hishigata Mikihiko's research team, several mysterious objects, Out-of-place artifacts or OOPArts for short, were taken from outside of Academy City to potentially help them after they got stuck:
The funerary Jade Mask (ヒスイの仮面,Jade Mask?) is also from Kʼinich Janaabʼ Pakal's tomb, being his funeral mask. Jade as a material held great significance in Mesoamerican cultures like the Mayas as it symbolized life and death, the sun and the wind. Therefore the deceased had jade beads placed in their mouths, as it was thought the jade absorbed the soul and aided it towards the afterlife.
The Baghdad Battery (バグダッド電池,Baghdad Battery?) is a set of artefacts consisting of a ceramic pot, a copper tube, and an iron rod. It's speculated this might have been the first instance of a galvanic cell, the most simple form of a battery, but it does not function as such due to missing several functions an actual galvanic cell uses. While its actual use is still unknown, the hypothesis that it's the first battery is still often thrown around as evidence for the idea that ancient civilizations used to have advanced technology that were forgotten with time, usually tying back in with the ancient astronauts hypothesis.
The stone spheres of Costa Rica (コスタリカの石球,stone spheres of Costa Rica?) are over 300 large, stones spheres buried in the grounds of Costa Rica, presumed to have been created by the Diquís culture. Their exact significance is still unknown, having only been discovered in the 1930s, though it's speculated they were markers for important buildings. Wild speculation widely varies from being created to mimic the cannonballs of the gods (from a god's blowpipe) to originating from Atlantis.
The Sakafune-ishi (酒船石,Sakafune-ishi?) is a stone slab located in the Japanese village of Asuka, where various archaeological carved stone slabs can be found as part of larger archaeological sites. This particular slab has its purpose still unknown, though it is speculated to have been used as a tool to create sake by separating and combining liquids flowing through the grooves. This theory is reflected in its name, as it literally means "Sake-Vat Rock". Other theories state it could have been used to create medicine instead, part of a garden facility (due to jars and pipes used to draw water found nearby), or even an astronomical chart.
Orichalcum is a historical metal that was described in ancient Greek texts, notably by Plato, which described it as second only to gold in value and mined in the legendary Atlantis, though the material now no longer exist. It was speculated to be either platinum or an alloy of several metals, due to the original Greek term translating to "mountain copper". In 2015, ingots were found in a shipwreck off the coast of Sicily, consisting of an alloy of copper and zinc, with several smaller percentages of nickel, lead, and iron, and it is now widely believed to be this historical orichalcum.
Hishigata Mikihiko also mentions taking extracts from cryptids, rumoured creatures in Urban Legends, but they aren't shown.
Mimosa is used heavily during this episode. Its meaning as "friendship" according to the language of the flowers, is even shown.
Differences in Adaptation
Esther's past with the Hishigata siblings is considerably expanded upon, but also differs considerably from the circumstances depicted in the manga.
In the original manga, Esther is implied to have allowed people to die during the experiments or at the very least allowed corpses to be desecrated, their brains removed from their body. In this episode, Esther is complicit with the use of live humans, specifically, women who are so in debt they'll volunteer for it, to be used for the experiment to the point that they lose their sense of self, essentially mind broken. Esther says that she is used to tragedy as she is a necromancer, and only concerned that Hirumi might be participating in the experiment.
The circumstances of how exactly Hirumi died in the original manga was never explored. Here in this episode it is revealed that she committed suicide.
The episode takes away Esther's agency, having Taowu possess Hirumi's body on it's own without Esther performing a ritual to insert the spirit into her body, as with the case in the original manga.