CASUALTY will be airing the first part of its Christmas two-parter this week - and fans will be time-jumping from December 2006 to December 2020.
The two special festive episodes - written by Barbara Machin - will see a troubled face from the past return for revenge.
Meanwhile, Charlie fears for his health and Iain will find himself in danger.
Here's a full collection of the seven biggest moments coming up.
1. Tess returns
It's Christmas 2020 and Charlie, who is still grieving, is preparing for the first one on his own without Duffy.
He is thrown when Tess - played by Suzanne Packer - arrives, having picked up ED shifts over the Christmas period.
Despite their once-solid friendship, there is palpable tension between the pair and Tess continues to pick up on Charlie's awful mood throughout their shift together.
2. Charlie struggles to cope
As Charlie's strange behaviour intensifies, it becomes increasingly evident that the COVID crisis is taking its toll on him.
When Dylan announces that they're running out of oxygen canisters, Charlie loses his temper and snaps at heavily pregnant HCA Rosie, who's panicking after hearing that her mum is looking for her.
Next week's episode will flashback to an episode that was broadcasted back in 2006.
That's because, unbeknown to Charlie, Rosie is the daughter of Laura Merriman, who brought Rosie into the ED fourteen years ago demanding treatment.
She was revealed to be suffering from Factitious Disorder by Proxy - previously known as Munchausen's.
Desperate to ensure that her mum can't reach her, Rosie panics and reaches out to both Charlie and social worker Adi, but is brushed off.
4. Tess makes a worrying discovery about Charlie
Back in present time, Tess uncovers the reason behind Charlie's distraction when she overhears a radiologist asking Charlie to attend a second brain scan.
Regardless, Tess is unconvinced that there's anything to worry about just yet and attempts to reassure Charlie that his symptoms - headaches, nausea, blurred vision - could simply be due to stress or exhaustion.
5. Rosie has a devastating accident
Laura admits herself as a patient under a false name - and eventually tricks Rosie into meeting her in the bathroom.
Confronted with her childhood trauma once again, Rosie is devastated to realise that her mum hasn't changed at all - still believing that she did the right thing fourteen years ago.
When Laura mentions Rosie's unborn child and how they can be a happy family, Rosie is frantic.
She makes another attempt to reach out to Charlie, but he's too distracted by his own personal problems and doesn't remember her.
The situation intensifies when Laura tracks Rosie down in the stairwell. They have a tense standoff and Rosie ends up falling over the bannisters.
6. New romance for Iain
Before Iain returned to the ED in April, viewers will find out that a Christmas romance blossomed between him and his HEMs partner Chrissie last year.
Unfortunately, their happiness is short-lived when they're called out to an RTC at a ravine, where a van has toppled over onto an unstable ledge that could give at any moment.
Determined to help, Iain ignores Chrissie's pleas to wait for backup, and abseils solo down to the van.
7. Iain puts himself in danger
After reaching the van, Iain is alarmed to find Zamir inside, who appears to be afraid of the driver, Peterson.
Iain secures Zamir in the back of the van and begins to treat Peterson, who opens up about his life as the husband of a refugee.
However, when Zamir objects to Peterson claiming him and his mum are married, Peterson loses his temper and launches himself at the child. This causes the van to collapse through the ledge and tumble down the ravine, with Iain inside.
Casualty airs these scenes on Saturday, December 4 on BBC One.