Mercedes is currently awaiting trial after framing Mitzeee Minniver for attempted murder and staging a fake kidnap of her baby son Bobby. The all-important court scenes will arrive on screen in early December.
Viewers will see Mercedes's cunning solicitor Jim McGinn advise her to blame Riley for everything, pointing out that it's her best option as the dead footballer isn't around to defend himself.
Although a vulnerable Mercedes is reluctant to take Jim's advice, she eventually gets on board with the idea and tells a shocked courtroom that Riley was the one who stabbed her, painting a picture of him as an abusive partner.
Jennifer Metcalfe, who plays Mercedes, told All About Soap: "That stuff was great to film - the writers came up with some brilliant material. The drama queen is back! It was challenging as well - she completely breaks down and says how bad Riley was. From the audience's point of view, it's really going to be quite shocking."
Asked whether Mercedes feels guilty, the actress replied: "Yes. She does feel guilty about it, but only as guilty as Mercedes can ever feel. So she gets over it very quickly!"
Fans can also expect to see Doctor Browning (Joe Thompson) back for the trial scenes as he takes the stand to give his version of events.
Although viewers know that Browning murdered Lynsey Nolan, one false suggestion made during the courtroom deceit is that Riley could have been the real killer.
On how Mercedes feels about seeing Browning again, Metcalfe revealed: "It's a total surprise. I don't think she knows how she'll react until she actually sees him, and when she does they're back in that moment when he was dragged away from her. She really did fall for him hook, line and sinker."
Hollyoaks airs weeknights at 6.30pm on Channel 4, with first look screenings at 7pm on E4.