WARNING: The following article contains spoilers for Season 1, Episode 7 of Dr. Stone, "Where Two Millions Years Have Gone," now streaming.
After Senku's physics-assisted rescue of Kohaku at the end of Dr. Stone's last episode, our hero claims his reward from her. Not by way of anything romantic, as the fearsome heroine makes clear, but by way of her allowing him passage to her village -- a potential source of able minds and bodies for Senku to use if he wants to stand a chance against Tsukasa's planned "Empire" of the young.
Senku is also fascinated by the idea of human survivors breaking free of their stone casing decades before he did, with questions about how long they've been active and the civilization they've built in the interim. Unfortunately, the teen science whizz can only glimpse these answers from the village's outskirts. The two guards, Kinro and Ginro, don't let him in on the community's strict "no outsiders" policy. This instantly begs the question: what outsiders? But Senku is too absorbed in trying to curry favor with them to think about this for the moment.
Naturally, he turns to the tricks of his trade. In the eyes of Kohaku's generation, the simple creation of soap bubbles is grounds for accusations of witchcraft, though thankfully Senku's "sorcery" is greeted with wide-eyed wonderment instead of pitchforks and torches. It also attracts the attention of Chrome, a young man who lives on the fringes of the village with a similar passion for science.
Both excited and threatened by Senku's presence, Chrome is quick to challenge his new rival to a "duel of sorcery," hoping to humiliate Senku and keep his own status secure. His first move is throwing a bunch of substances into a fire to make it grow in intensity and change color, something he calls "Rainbow Bridge." He earns some impressed noises from his spellbound audience, but Senku isn't one of them. Chrome is flabbergasted when the newcomer not only correctly guesses what materials he threw in the fire but also their place of origin -- the cave that Senku has been sourcing his own ingredients from. He also sees through Chrome's static electricity trick, too.
A desperate Chrome then raises the stakes, betting the contents of his entire treehouse laboratory on his victory in a test of numbers. If Senku loses, he has to leave the village and never return. Senku agrees, and then promptly annihilates the "sorcerer" with his knowledge of complex equations.
Chrome is deflated, but despite his crushing defeat, Senku isn't unimpressed with the aspiring student of science. Rather, he feels relieved that, even in his possible absence, humanity's need for learning and self-improvement through technology and innovation would have still been irrepressible. Chrome shouldn't feel too disheartened, either, since Senku's brag that he has "two millions years of human history" stored in his brain is also an admission of the hugely unfair advantage he had going into the duel.
Chrome is too transfixed by Senku's info-dump to point this out, even being brought to tears by the revelation that all of the advancement of Senku's original time period was brought to a shuddering halt 3,700 years ago.
While this is all going on, Kohaku completes the mission she was sent out to do before she stumbled across the showdown between Senku and Tsukasa and retrieves "healing" water from the hot springs for her older sister, Ruri, a villaage leader who suffers from a chronic, unspecified illness. (Basically, she falls over and coughs a lot.)
Hearing about Ruri's plight from a concerned Chrome -- who has been trying and failing to cure her -- Senku pledges that the two of them will engineer something vital to not only her health, but the health of every human: antibiotics. That's not a bad way for Senku to ingratiate himself into the xenophobic village, either.
New episode of Dr. Stone air every Saturday night as part of Adult Swim's Toonami block, and on Crunchyroll.