I have not seen the movie (although I really want to), so I went in reading this with no knowledge. This, I'm assuming serves as a prequel to the movie as it is about Hiro and Tadashi's childhood.
Hiro is crowned a prodigy and is bullied by other kids because of it and slowly starts to become a Hikikomori. Tadashi wants to help his brother so he starts to invent things. These inventions eventually lead to Hiro opening up again. It ends with Tadashi creating Baymax for Hiro.
The artwork for this was really well done. Both Hiro and Tadashi looked super cute and so did Baymax. And this has really made me look forward to watching the movie. Tadashi is probably the greatest older brother of all time.