The shocking incident comes after the glamorous love rivals have a huge row, which is sparked when Mitzeee discovers that Mercedes (Jennifer Metcalfe) has been her stalker all along.
When Riley and Lynsey find Mercedes in a desperate situation and Mitzeee with blood on her hands, it seems that the feud between the two women has taken a disturbing twist. Has Mitzeee done the unthinkable, or is someone else to blame?
Digital Spy recently caught up with Rachel Shenton, who plays Mitzeee, to hear her thoughts on the dramatic storyline.
How does Mitzeee feel when she finally gets together with Riley in the next few days?
"She's so happy. Mitzeee really wants that unit of her, Riley and Bobby. It's very unconventional, but she loves it. She loves Riley for all the right reasons and I think, for the first time, she's actually quite content."
Mitzeee, Lynsey and Mercedes have all been battling for Riley's affections. Do you think Mitzeee is definitely the best match for him?
"I think so - I think Riley was always going to be with Mitzeee in the end. Mitzeee knew that she had more sway with Riley, because they go back such a long way and there is a history there.
"Ultimately, they had a strong connection and I think there's no effort required for Mitzeee and Riley to be together - they do it so easily. They live together at the flat, they do the nappies and the dinner together, so it all works fine. I think Mitzeee was always going to get him, but her question was always whether Riley felt the same way or not."
We've seen that Mitzeee and Riley have their fans, but some viewers can't get past their family connection! Do you like the fact that they cause a bit of a debate?
"I do like that people are talking about it, yeah! As actors, when it first came up for the two of us, we said the same thing - 'How does that work?!' But it isn't incestuous, because they're only second cousins. Also, they grew up separately and came back into each other's lives as adults. It certainly doesn't feel like they're family, and it doesn't feel seedy. But yes, I do like that everyone talks about it!"
With Mitzeee and Riley together, Mercedes returns to her old tricks. How does Mitzeee react when she realises that her stalker is back on the scene?
"I think at first, Mitzeee is so happy with Riley that she feels like she's finally turned a corner and she can put everything to bed with the whole stalker thing - it's been a while since that reared its ugly head. She's very content, but then strange things start happening again.
"Mitzeee comes home one night and there are photographs of her all over the flat, which have been taken when she's out and about. Mitzeee is shocked that this person not only has been following her every move, but also has a key to her flat or has gained access in some way. So not only has the stalker come back, but they've come back on a whole new level."
Does she have any theories on who the stalker might be?
"Mitzeee definitely has no idea that it's Mercedes - in fact, Mercedes is probably the last person that she'd suspect. She actually thinks that it's Lynsey and accuses her, because it seems to make sense.
"From Mitzeee's point of view, Lynsey isn't going out with Riley anymore and could be slightly bitter about it. Lynsey would also potentially still have a key to the flat, so it seems like the most logical explanation."
What can you tell us about Mercedes's stabbing?
"Mitzeee comes home and she's in such a state because of the photographs and the other things that the stalker has done. She takes some antidepressant tablets to calm her down, and she has some wine too - which isn't a great combination!
"Then Mercedes comes round saying that she wants to see if Mitzeee is okay. Mitzeee still has no idea who the stalker really is, but as they're talking, Mercedes says something that makes Mitzeee discover the truth - and she's totally shocked.
"Mitzeee is petrified and picks up a knife in the kitchen. The next thing you know, Mitzeee wakes up after being unconscious due to the tablets and the wine. The first thing she sees is Mercedes with a stab wound and there is blood everywhere. Mitzeee doesn't have a clue whether she could have stabbed Mercedes, or if someone else came into the flat and did it."
Did you ever imagine that this story would take such a dramatic twist?
"I certainly didn't expect it to play out in the way that it has. But it's just so exciting for me to film and I feel like Mitzeee has come on such a journey. We saw her as a bit of a bimbo and an airhead when she first joined the show, but then people had a bit more time for her and realised that she's a decent person underneath it all.
"As an actress, it's just so great to be able to play something like this and get your teeth into it. I've been blessed that I've been given so much, and these scenes were just a joy to film with Jen."
Mitzeee turns to Brendan for some help, doesn't she?
"She does! Mitzeee and Brendan formed an alliance quite early on, and they've been friends for a long time. He's her partner in crime, as he's the one who helped to send Warren down and the person who she rings when she's in trouble. It's very natural that Brendan would be the first person she would go to, as she knows she can trust him.
"Brendan is there for Mitzeee and he listens to her, as she still has no idea of what's gone on. He helps her as much as he can, but I'm not allowed to give away too much about what happens after the stabbing…"
There's a lot of drama at the moment, but are you still hoping Mitzeee and Riley get a happy ending?
"I certainly hope so! I know it's what she wants, but things happen and in true soap style, it's very rarely plain sailing. But it would be nice as I think Mitzeee deserves a bit of happiness. She deserves a good bloke!"
Would you like to see Seth back at some point?
"Yes, definitely! Miles [Higson] is a really good friend of mine and I think it'd be nice to see Seth back because Riley's in trouble at the moment. Maybe Seth could come back to check if he's okay, or to see the baby. Who knows, but we've got a great storyline team so I'll leave it up to them."
Are you excited to be in Hollyoaks Later again this year?
"I'm very excited and we've just started filming for it, actually. I love doing Hollyoaks Later because you're on location and it's a different sort of vibe. It's really enjoyable and the story this year is going to be great."
How's the training going for your Mount Kilimanjaro climb?
"The climb has been pushed back a year, so I'll now be doing it in September 2013. Because of my storyline, Hollyoaks can't release me to do the climb this year, which is fine because I'm very busy at work and really enjoying it. But I'm still training for it, so hopefully next year I'll literally jog up that mountain!"
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Hollyoaks airs weeknights at 6.30pm on Channel 4, with a first look screening at 7pm on E4.