Fans in Australia will see Sarah Beaumont return next Monday (February 4) to set the record straight about her mysterious 1998 affair with Karl Kennedy.
Speaking to The West Australian, Charles explained of the plot: "Back then she escaped by moving to the UK with Peter Hannay [Nick Carrafa]. They've had a couple of children who are young teenagers now and their marriage hasn't worked out and, for business reasons, Sarah is bought back to Erinsborough.
"She didn't actually want to come back but, because of her prior knowledge of Erinsborough, the company she works for asks her to complete the deal which unfortunately involves the Erinsborough News, which is where Susan works.
"No matter how hard Sarah tries to skirt around her past and be professional in her dealings in Erinsborough, the time arrives when you realise you have to get closure and sort things out with people."
Charles added coyly: "Sarah decides to revisit everything and fireworks erupt from that."
Meanwhile, the actress thanked producers for creating a "grittier" storyline for the character, saying: "Fourteen years ago when Sarah left, it was extremely difficult and complicated but it wasn't investigated back then as much as it should be.
"At that time, storylines were only touched on and not delved into. The way they've written her back in is so good. It's grittier and more real, which is something they've wanted to do with the script these days.
"I was just flabbergasted when I opened the pages and saw some of the things that were actually spoken about in the scenes - about whether Sarah and Karl slept together and really got down to the nitty-gritty."
Neighbours airs weekdays at 1.45pm and 5.30pm on Channel 5 in the UK, and weekdays at 6.30pm on Eleven in Australia.
> Read Digital Spy's recent interview with Nicola Charles
> Neighbours Alan Fletcher: 'Karl won't give up on Susan'