The exes spent the night together while filming in Marbella last month and we'll find out on Sunday night just how that happened and the aftermath.
We already know, however, that Lydia isn't happy…
A preview clip shared by ITVBe shows Lydia confessing to close friend Chloe Lewis that she's mortified by the events of the previous night.
"I'm never going to drink alcohol ever again, I feel horrific," lamented Lydia, prompting Chloe to say: "What went down? I feel like there is more to it than just a hangover."
"I can't even say it…" said Lydia.
"Does it begin with an A?" replied Chloe.
Oh, Chloe, spot on!
Looking sheepish, Lydia replies: "He basically ended up staying. Oh God. I just feel so s**t about the situation. It's the worst mistake of my life."
Lydia and Arg split earlier this year after an on-off romance spanning seven years. They broke up off camera, which was a surprise to fans as they'd just been seen on screen house hunting together.
News of their night together came earlier this week when a source told Reveal: "[Arg] has always struggled to let her go and the pair ended up hooking up for one night only."
Arg later confirmed he'd slept with Lydia in an interview with the TOWIE official Twitter account, saying: "It's been emotional. As per usual, people's heads have gone. My head has gone... it went after I stayed the night with the love of my life. My head went. I didn't know whether I was coming or going."
Viewers saw Lydia and Arg come face to face in Palma last series, where Lydia came under criticism for being "brutally honest" with Arg.
During a conversation about the breakdown of their relationship, Lydia told her ex-boyfriend: "You're not the kind of person I want to be in my life. I would like you to stay away from my friends and family... My life is my life now, and I'm getting on with it, we'll keep it that we're strangers. We will just walk past each other and that's how I would like it. I don't want to know you anymore."
Her words left Arg in tears, but Lydia later defended her comments: "Everything I said was completely from the heart and what I wanted to say at the time. I had to be quite brutal but that's the only way I could play it – it was all from the heart."
Lydia and Arg had appeared to clear the air by the end of the series, but not enough for Lydia to allow her co-star to hug her.
TOWIE returns Sunday 9 October at 10pm on ITVBe.
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