Ep. 4: Fire the Smoke Signal

After Senku creates a successful batch of gunpowder, which Taiju accidentally sets alight, the group spot a different source of smoke in the distance, suggesting that there are other survivors. Senku makes the decision to light the first further to use as a smoke signal, leading Tsukasa to find him and use Yuzuriha as a hostage to obtain the recipe for the revival fluid. As Senku relinquishes the recipe to save Yuzuriha and is given the choice of either abandoning science or dying, he recalls how he developed his love of science and became friends with Taiju. Refusing to abandon science, Senku prepares to die at Tsukasa's hands. (Source: Wikipedia)

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