WARNING: The following contains spoilers for Season 2, Episode 4 of Dr. Stone, now streaming on Crunchyroll.
In Dr. Stone, tensions are escalating between Senku’s Kingdom of Science and Tsukasa’s Empire, and the stakes for Senku are even higher in Episode 4. After a blunder from the cell phone deployment team, Tsukasa takes one of the members as a hostage. That hostage is none other than Chrome, effectively stripping Senku of his right-hand man. This sequence of events prompts Senku to take a more direct measure and move his base closer to Tsukasa territory with hopes of saving Chrome. And what better way to do this than with the development of another great Senku invention: the car.
When Chrome, Magma and Gen delivered the cell phone to Senku’s gravesite earlier this season, they triggered one of Tsukasa’s defenses -- an archer named Ukyo. He is a cunning and skilled member of Tsukasa's army who is known for his exceptional hearing. It is revealed that before the petrification, Ukyo used to be a sonar operator on submarines, which resulted in his incredibly sensitive hearing.
In last week's episode, Ukyo eventually tracked down the three before they could leave Tsukasa’s territory. Chrome used a smokescreen distraction, with Magma’s help, to allow Gen to escape and make it back to Senku for the phone call with Nikki.
Season 2, Episode 4 takes place after the phone call, and Chrome and Magma are still in a standoff with Ukyo. During this standoff, Chrome reasons the best way to keep the rambunctious Magma and himself from getting killed is to surrender. Magma runs back to tell the village, and Senku makes one of the most important decisions in the war. Considering Homura is already restrained, Senku decides it’s time for the Kingdom of Science to move its operations closer to Tsukasa. They have no hope of saving Chrome with small units rushing in. They’re going to need to apply greater pressure and speed up their efforts in order to save him.
Using various items from around the village, such as the furnace, honeycomb, brass, and bamboo, Senku and the villagers build a stone age automobile, capable of transporting the operations to just outside of Tsukasa’s Empire. While Senku is having an extreme amount of fun manufacturing his modern machine, Chrome’s future still hangs in the balance.
Ukyo brings Chrome straight to Tsukasa and the highest figures in his army. With Chrome dangling above a waterfall and steep drop, Tsukasa offers an ultimatum: bring him Senku’s head and no one will be hurt. Chrome’s love for science, strong resolve, and loyalty to Senku lead him to decline, only to be dropped off the cliff seconds later, and seconds after that saved by Tsukasa. Ukyo pins Chrome to a branch with his arrow, Tsukasa throws a whole tree trunk under Chrome’s feet as a bridge. Tsukasa won’t kill him just yet, and instead asks Ukyo where he found Chrome meddling around. Ukyo lies about Chrome’s whereabouts, never mentioning Senku’s gravesite.
Ukyo’s lie is intriguing. It is important for Senku to maintain three advantages against Tsukasa: technology, information and communication. The new car is just the latest evidence of Senku's technological superiority, but his other two advantages are in jeopardy. Kohaku presumes Chrome is likely to let some secrets slip, which could prompt Tsukasa to be even more suspicious of Taiju and Yuzuriha. For now, Tsukasa doesn’t know about Chrome’s true objective. It’s unclear why Ukyo lied. Whatever plans Ukyo has may have bought Senku time, and what better way to speed up a rescue than a transportation vehicle.
Even without his chief science assistant, Senku is still able to bring modern technology to the stone age, but it’s clear that Chrome is an essential piece to the Ishigami Village both in science and community. This rescue mission might be risky for the war effort, but Senku and the rest of the village have a strong bond with Chrome and a responsibility to save him.