Viewers will see Kat return to Walford later this week, but her problems with husband Alfie are far from over as she starts to fear that the spark in their marriage has fizzled out.
Kat (Jessie Wallace) tries her best to get things back on track with Alfie (Shane Richie), but he fails to realise that she wants the passion back in their relationship.
As Kat isn't getting the attention that she craves at home, she later finds it impossible to resist when someone else starts showering her with affection.
The Queen Vic's landlady is plagued with guilt once she starts cheating on Alfie again, knowing that she is making a huge mistake. However, Kat's attempts to call off her affair prove futile as she cannot resist her new boyfriend.
EastEnders chiefs have confirmed that this will be a long-running storyline, leaving viewers guessing over who the mystery man is for quite some time.
At least five names are in the frame, including Ray Dixon, Michael Moon and Branning brothers Derek, Jack and Max.
Even the BBC show's cast and crew will be left in the dark over the storyline, as all scenes featuring Kat and her lover will be filmed with an extra to prevent leaks.
EastEnders' executive producer Lorraine Newman commented: "The consequences are not only great for Kat and Alfie, but also for each individual suspect. The audience will join the cast and crew guessing across the summer and autumn as we eliminate the suspects one at a time, leading to one almighty explosion."
EastEnders fans will see the storyline begin on screen later this month.
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