After finding out they are expecting a baby, a young couple escape from north Wales to start a new life in Cardiff. Budding detective Sally Morris is employed by the frantic family to look for their daughter, starting a dangerous adventure to the dark underbelly of the city, west Wales tourism, and, ultimately, to the copper mountain's rotten core.

Ar ôl darganfod eu bod yn disgwyl babi, mae cwpwl ifanc yn dianc o ogledd Cymru ac yn dechrau bywyd newydd yng Nghaerdydd. Caiff y darpar dditectif Sally Morris ei chyflogi gan y teulu gofidus i chwilio am eu merch gan ddechrau ar antur gyffrous a pheryglus sy'n ei harwain i isfyd tywyll y brifddinas, diwydiant twristiaeth Aberaeron ac i galon lwgr y mynydd copr.