Katja, a young girl from Germany, finds a letter in Welsh sent to her great, great grandfather by a friend from Wales while she explores her mother's papers. To learn more, she visits Wales during the National Eisteddfod in Llanrwst. As the letter leads her to the slate-quarrying area of Blaenau Ffestiniog, Katja learns more about family's life and secrets.

Mae Katja, merch ifanc o'r Almaen, yn darganfod llythyr yn y Gymraeg a anfonwyd i'w hen, hen daid gan gyfaill o Gymro wrth fynd drwy bapurau ei mam. Er mwyn ceisio darganfod mwy, penderfyna ymweld â Chymru ar achlysur Eisteddfod Genedlaethol Llanrwst. Wrth i'r llythyr ei harwain i ardal chwarelyddol Blaenau Ffestiniog daw Katja i ddysgu mwy am fywyd - a chyfrinachau - ei theulu.