Grandpa Oskar takes neither the truth nor himself very seriously. Even after a prison sentence for imposture, the senior sees little reason to completely improve. First of all, he needs a roof over his head - and with his daughter Tilda, who still resents the last escapades, the ex-con expects little chance. Unfortunately, his only place to stay, an old construction trailer, is with his daughter in the garden. There he meets his autistic granddaughter Fanny after the first secret night, who - unlike him - takes the truth very seriously. In order to gain her trust, Oskar does not introduce himself to the comic-loving eight-year-old as the Knacki-Grandpa, who has been missing for years, but as Professor Krypton. For Fanny, who prefers to wear a brightly colored superhero costume and is teased at school for it, the omniscient mentor from the Superman planet comes just as requested. Even Tilda, who soon notices what is going on in the garden, falls back on Oskar. In order to be able to attend further training, she entrusts him with her daughter for a week. With her outsider grandpa by her side, Fanny is tackling her big dream: She wants to win the talent competition at school - as a Fanny Supergirl, of course.