Tonight's (September 14) emotional episode of the soap saw Kat tell a surprised Michael (Steve John Shepherd) that she has been cheating on Alfie, showing that he cannot be the mystery guy in question.
The plot development leaves Branning brothers Derek, Jack and Max as the remaining three suspects in the ongoing storyline. Queen Vic chef Ray Dixon was previously ruled out a few weeks ago.
Scenes broadcast this evening showed Kat (Jessie Wallace) calling her secret boyfriend in an attempt to end the affair once and for all. The pub landlady appeared desperate to save her marriage after finally being honest with husband Alfie.
However, as expected, there was no firm answer on the identity of Kat's lover, as it was impossible to tell which of the Branning men she was contacting.
Kat and Alfie later headed off for a break to work on their marriage problems. However, the pair won't be off screen for long as their return episode airs on October 1.
Michael, meanwhile, has troubles of his own ahead after his new wife Janine decided to leave him and their baby daughter Scarlett in the latest episode. Charlie Brooks, who plays Janine, is now on a six-month break from EastEnders.
EastEnders continues on Monday (September 17) at 8pm on BBC One. Get the Inside Soap magazine on your iPhone or iPad