Spartan Crafts Club.
Using the new waterwheel, Senku mechanizes the ironmaking process, freeing up the villagers to prepare for winter. Meanwhile, Senku, Chrome, and Kaseki begin developing light bulbs, allowing them to celebrate Christmas by decorating a tree. Chrome uses the new invention to explore deeper in caves, where he finds a host of new minerals. Senku and Kaseki attempt to create a vacuum tube, but they cannot find a filament that can withstand the heat. During a New Year's sunrise, Senku receives inspiration from Suika and realizes tungsten is the solution and employs himself, Chrome, and Magma on a spelunking expedition.