战后复兴的东京，、少年侦探金田正太郎和他的“铁人28号”一起解决了一个接着一个的困难事件。但是在战争中行踪不明的正太郎的哥哥出现在了正太郎面前。（正太郎的哥哥也叫正太郎，故下面用SHOUTAROU代替）。为了战争而接受铁人操控训练后返回日本的SHOUTAROU却得知了博士的遗言为“将铁人留给弟弟正太郎”。而现在的日本也已恢复了和平，他的存在似乎变得毫无价值。但SHOUTAROU还是接受了这个现实，并向弟弟正太郎传授铁人的操纵技术。与此同时，东京埋藏着“废墟弹”的事实也被暴露了出来，那是铁人28号的开发者金田博士研制出来的最终兵器。如果这个武器爆炸的话，东京将化为一片灰烬。SHOUTAROU传达了金田博士的另一个遗言：废墟弹的所在地只有铁人才知道……。 　　于是正太郎一行人开始积极准备废墟弹的回收处理工作。但却突然出现了一个名叫“白昼的残月”的复员兵人。他指责正太郎没有持有铁人的资格，并不断的企图杀死正太郎，干扰回收工作。 　　现在，被铁人和废墟弹掩盖起来的可怕事实正在慢慢变得清晰……

Japan, ten years after World War II. Boy detective Shotaro solves mysteries and fights crime with the help of Tetsujin 28, the 50-foot tall giant robot left for him by his late genius father. But one day he's shocked to learn that his father also adopted and raised another boy named Shotaro, now a repatriated soldier. Just as surprising to our young hero is the fact that his “big brother” can also control Tetsujin, even better than he can. But with the emergence of this elder brother comes danger, as a mysterious assassin, Morning Moon, closes in on the boy for being unworthy of controlling the robot. Meanwhile, the capital faces destruction when multiple bombs are discovered, planted by the two Shotaros' father himself!

戦後10年を迎えた東京で、少年探偵の金田正太郎と鉄人28号は、戦争で使われた武器の不発弾を狙うロボットと戦闘を繰り広げていた。そんなある日、生き別れになっていた義兄のショウタロウが現れた。金田博士の元で鉄人の操縦士として訓練を受けていたショウタロウは、見事に鉄人を操り敵を倒していった。

2차 대전이 끝난 지 10년 후. 소년 탐정 가네다 쇼타로는 거대 로봇 철인 28호를 조종하며 범죄를 해결해 나가던 어느 날 자신이 그의 형이라고 주장하는 인물을 만나게 된다.