Zatch Bell!

Another Zatch (1x21)

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Zatch and Kiyo head towards the forest where Zatch was found. While crossing through a village, the villagers retreat in fear from Zatch's presence. On the way, they meet Megumi and Tia who reveal that they saw someone who looks like Zatch. In the forest, Zatch attracts snakes with his whistle but is saved by Professor Dartagnian in a fairy suit. Dartagnian reveals that he has been researching the forest since an explosion six months ago. Kiyo and Zatch investigate the site of the explosion. Once there, Zatch suddenly starts to glow gold and when he regains consciousness, regains his memory about his life in the forest. Zatch reveals that another Mamodo who looked like Zatch, attacked him and caused him to lose his memories. Dartagnian who was witnessed the golden light reveals that Kiyo's book was glowing also. Kiyo opens the book and realizes he can read the fourth spell.