Tra bo Julián ar y trên adref gyda’i Nana, mae’n sylwi ar tair menyw sydd wedi’i wisgo’n anhygoel. Mae’u gwallt yn bilio mewn lliwiau disglair, eu ffrogiau’n gorffen mewn cynffonau pysgod, a’u llonder yn llenwi’r cerbyd trên.

Pan mae Julián yn cyrraedd adref, mae’n breuddwydio am beth welodd ar y trên; yr unig beth mae’n gallu meddwl amdano yw creu ei gwisg môr-forwyn ei hun a gwisgo fel y merched arall. Ond beth fydd ei Nana’n meddwl am y llanast mae’n ei greu – ac yn bwysicach fyth – beth fyddai’n meddwl am sut mae Julián yn gweld ei hun?

While riding the subway home with his Nana one day, Julián notices three women spectacularly dressed up. Their hair billows in brilliant hues, their dresses end in fishtails, and their joy fills the train carriage.

When Julián gets home, daydreaming of the magic he’s seen, all he can think about is dressing up just like the ladies and making his own fabulous mermaid costume. But what will Nana think about the mess he makes – and even more importantly – what will she think about how Julián sees himself?