Speaking to What's on TV, the actress revealed that Scout is heartbroken after she realises her mum is financially benefiting from a credit card scam.
McGlynn explained: "Scout thinks she's in love with Danilo and wants to marry him, but he's only using her for a visa!
"Scout's mum is encouraging her to marry Danilo, and Scout thinks it's because her mum wants to help him stay in the country.
"She thinks her mum's encouraging her because she loves her, but it's just not the case - she's actually trying to make money out of her."
The star added: "It is really sad. Scout actually thinks her life is going pretty well, her mum seems to care for her and she believes she's in love with Danilo. In the end Scout's whole world comes crashing down.
"In the end Scout realises what her mum is like and gets some justice. It's a real turning point for her. She's growing up, which is good!"
Praising Waterloo Road's scriptwriters, McGlynn added: "I sometimes get taken aback when I look at the scripts; they're often quite emotional."
> Read the full synopsis for Wednesday's Waterloo Road
> Waterloo Road stars launch new Scotland school