Twelve-year-old Nano witnesses a sinister act: evil Professor Schlotter injects Nano’s Grandpa with a Gobot, a tiny robot that makes it possible to remotely control its victim. Nano goes to their family doctor, Dr. X, for help. But incredible things are happening in Dr. X’s practice as well: together with genius scientist Micro Minitec, the doctor has developed a miniaturisation procedure with which he shrinks a submarine and sends it through the human body on a journey of medical discovery.

Nano and his friend Lilly are miniaturised and embark on the risky journey into his Grandpa’s body to defeat the Gobot. But Professor Schlotter and his henchman Serge will not surrender easily. They storm Dr. X’ practice and overpower the scientists. Now Nano and Lilly are stuck in Nano’s Grandpa’s body, left to their own devices. They have to face not just the Gobot, but also dangerous killer cells, searing stomach acid and the confusing maze of the brain. Time is of the essence because the oxygen supply in the submarine is limited. It’s a good thing that Dr. X’ submarine pilot, the talking pink rabbit Rappel, has an axe to grind with Professor Schlotter…