Otoi Nobuhiko's family is full of robot engineers: both his mother and father are renowned scientists, and his grandfather, Professor Otoi, is world-known for his amazing human-form robots called the Atrandom Numbers. So when Nobuhiko is sent to live with his grandfather while his parents work abroad, it's no surprise to see his grandfather working on his latest model, Signal. His grandfather agrees to program Signal to Nobuhiko's liking, but due to a seemingly innocent occurrence during the process, instead of the big brother that Nobuhiko wanted, he gets a big brother and little brother in one! Now whenever Nobuhiko sneezes, the cool and arrogant Signal shrinks into an innocent and mischevious 2 year-old! If that wasn't enough trouble, more and more of the Professor's robots begin to show up.

Twin Signal starts out as rather childish and light-hearted, but within a few volumes the pace turns into more serious action as the Atrandom Numbers defend the Otois and themselves against rivals who seek the Professor's latest research.