For the official details of the storyline, which is set to run until the autumn, check out the main story in the news section.
Here, though, Soap Scoop takes a look at the five suspects who have been named by EastEnders chiefs. Which of them will tempt Kat to cheat in the coming months?
Ray had a previous relationship with Bianca Butcher years ago and has recently begun dating Kim Fox. The pair seem happy together, but Kim's struggles to bond with Ray's daughter Sasha have also caused some problems between them.
Ray has so far appeared to be a decent family guy, so may not be the top suspect among EastEnders fans. However, viewers have also seen that he is an expert at flirty banter, while his job as a chef at The Vic would make him perfectly placed for a secret fling with Kat.
Would Ray risk things with Kim, not to mention his position at The Vic, by pursuing the pub landlady?
Michael is due to marry Janine Butcher later this month, and the couple are currently expecting a child together. Although Michael and Janine's similar natures have made them a good match, it's never been quite clear whether there is a genuine love between them or if Michael is more interested in his fiancée's fortune.
Michael and Kat have already had a previous fling, which led to Kat falling pregnant with Tommy Moon. While Michael has nothing to do with his son, it's possible that he still carries a torch for Kat.
With Michael and Kat set for a showdown over Jean in the coming days, it'd be a surprise if their tension turned to passion - especially as Kat knows how scheming he can be. But could Michael turn out to be the secret lover after all in a shock twist?
Max is currently living with his ex-wife Tanya and their three children. Viewers will know that Max has a history of affairs and recently struggled to resist temptation when Roxy Mitchell took an interest in him.
Max currently seems happy now that he's back with Tanya, but they have also struggled to get things in the bedroom back to normal in the aftermath of Tanya's battle against cervical cancer.
Show bosses have also promised a huge year for the Brannings, and it's known that Jo Joyner - who plays Tanya - will be taking a break from the soap in 2013. Could the fallout from a Max and Kat affair lead to her temporary exit?
As the baddest Branning brother, it's unlikely that Derek would have any qualms about embarking on an affair with Kat, despite the fact that he and Alfie struck up an unlikely friendship earlier this year.
Since returning to Walford last autumn, Derek hasn't had a serious relationship, so perhaps EastEnders chiefs have decided that it's time he had some romance.
Kat has never been afraid of living dangerously, so could she turn to bad boy Derek to put some spark back into her life?
Jack has been single for some time in the aftermath of his divorce from Ronnie, so the Walford businessman is another character who may be ready for a new relationship.
Kat and Jack have always got on well, especially as Kat was so sympathetic to Jack when the truth about the tragic baby swap came out last year.
Jack has a reputation as one of Albert Square's biggest womanisers thanks to his past antics, so viewers may suspect that he'd be well up for a steamy fling with Kat. Will she be his next love interest?
EastEnders have named Ray, Michael, Max, Derek and Jack as the main suspects, but they've also said that there will be "at least" five men in the frame - so could there be a further twist?
As we saw with Coronation Street's Frank Foster death mystery earlier this year, the culprit isn't always on the suspect list, so we wouldn't rule out an outsider. Would anyone else in Walford catch Kat's eye?
> 'EastEnders' lines up Kat Moon affair mystery
> 'EastEnders': Who will Kat's mystery lover be? - poll
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