The bonus show, titled PyC, will air for five short episodes on the S4C website next month.
Aimed at the 18 to 30 audience, PyC will show a different side to Cwmderi as it focuses on the younger characters.
PyC kicks off with established regulars Gemma and Sioned heading for a night out in the city, where they meet new friends Llew (Gwydion Rhys), Jess (Ceri Lloyd) and Kai (Rhys Downing).
Catrin-Mai Huw, who plays Gemma, commented: "Immediately we see that there is a connection between Gemma and Llew. The attraction is overwhelming, but this is no ordinary love story and things are far from simple."
She added: "Jess is Llew's sister and Kai is a character from their past. They all influence Gemma and Llew's love story and bring hurt and happiness to the couple in their turn."
The PyC website will launch on Tuesday, April 2. From Monday, April 8 to Friday, April 12, the five episodes will be released nightly at 9pm.
In a new method of storytelling for the S4C soap, the website will also link to fictional Twitter and Facebook accounts for Gemma, Llew and Jess, allowing viewers to see their interactions beyond the five episodes.
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