Dr. Stone season 4 episode 16 recap: What did Joel discover inside the Medusa? Here's what happened

Dr. Stone is heating up in its final arc, and episode 16 of season 4 delivers a solid mix of science, strategy, and suspense. As the Kingdom of Science continues their race against Stanley Snyder, they split into two groups, one focused on cracking the Medusa, and the other on trekking across the Andes.

This episode balances humor, technical insight, and looming danger perfectly, giving us new character dynamics and a few important reveals. From Joel’s genius return to Luna’s hilarious misunderstanding, this chapter is a fun but pivotal step forward in the road to the Amazon and the future of humanity.


Back in Corn City, Nikki and the rest of the crew begin studying the Medusa. After Homura retrieves it from the river, the group struggles to dissect it safely. Realizing they need a specialist, they revive Joel, the expert watchmaker, who quickly becomes their best shot at unlocking the device’s mystery.


Joel deduces that the Medusa’s power source is a diamond, much like a replaceable battery. Though the diamond runs out of charge, Joel is confident it can be reused. This breakthrough opens a major path forward in their plan to counter Stanley and restore the world.


Meanwhile, Senku’s team preps to cross the Andes Mountains. With Stanley still chasing them, time is limited. They weigh everyone for efficient bike usage, prompting some lighthearted moments, especially Luna’s assumption that she’s dating Senku, much to the group's amusement.

Once the mountain climb begins, the group struggles with the steep terrain. But true to form, Senku proposes building a ropeway to get through safely. It’s a classic Dr. Stone move: a clever, science-backed solution to a seemingly impossible challenge.


Aside from the science and tension, this episode also delivers some fun character beats. Luna’s romantic assumptions about Senku create a cute moment of levity. Chelsea’s confidence about the rainforest's condition also reassures the team and keeps spirits high.

These interactions help balance the fast-paced adventure with emotional connection, keeping fans engaged not just by the plot, but by the bonds forming between characters.