Eight-year-old Thaddeus Fong is insanely jealous of his baby sister and uses his intelligence as a weapon to prove his sister is really an alien or at least a portal for aliens to come through. He starts counting his sister's Ga (as this is the only sound she makes) and everytime it is a prime number. This leads him to conclude that his sister is an alien but eventually he discovers she is really a portal for aliens to come through.
So he uses his evil genius to corrupt the aliens into doing bad things like taking over the world but they keep doing good deeds like knitting socks and blankets. In a stroke of genius he manages to convince his parents that she is an alien and it works when she is taken away by the government to be studied. But surprising to him, Thad discovers that that his sister really loves him and he has broken her heart. So to make things right he shows his parents he is a portal to the aliens too so he can be with his little sister.
It is told in a delightful graphic book version. I didn't expect to really like it as much as I did. I read his other graphic novel "American Born Chinese" and enjoyed "Prime Baby" much more.