Episodes 4-6 of the Soukyuu no Fafner: Dead Aggressor - The Beyond series.

Tatsumiyajima is the central island in the middle of a small cluster of islands, in a sleepy backwater of the Japanese isles. Not much happens there, and the island's young people go to school knowing that their lives are likely to remain peaceful and undisturbed. Or so they have been taught... but the truth is different. The fate of mankind is on the line, and Tatsumiyajima is the last line of defense against a hostile and incomprehensible enemy. At the center of it all, fighting for Humanity's continued existence, is the giant robot Fafner, the dragon that guards this final treasure of mankind. (Source: ANN)

This theatrical version based on the manga by Yokoyama Mitsuteru, deals with the genius boy detective Kaneda Shotaro and his giant robot, Tetsujin 28, set in Tokyo ten years after the end of the Second World War. Shotaro's missing brother appears as a a new character who was trained as a pilot of the soldier robot Tetsujin which Japan had created for the Second World War. However, with the end of the war, there was nothing left for him but a peaceful Japan. In the meantime, Destruction Bombs are found in many places in Tokyo. The bombs were developed by Professor Kaneda, the creator of Tetsujin 28 and Shotaro's father, as the ultimate weapons to protect the mainland. If the bombs explode, Tokyo will be reduced to ashes.