The long-running mystery was finally resolved this evening (December 20) as Derek (Jamie Foreman) was confirmed as the man who Kat slept with throughout the summer.
After secretly following Kat (Jessie Wallace) to the bedsit on George Street in tonight's episode, Alfie demanded answers from his unfaithful wife over her affair.
Although Kat was reluctant to divulge any information, Alfie soon discovered that her mystery man was one of the Branning brothers when he caught a glimpse of the tenancy agreement that had been left behind at the bedsit.
Over at the Queen Vic shortly afterwards, Alfie furiously confronted the Branning siblings and eventually came to the conclusion that Max was the guy in question.
However, when a frantic Kat announced that Max wasn't her lover, Derek and Jack were left as the remaining two suspects still in the frame.
After Alfie urged Kat to give him an answer once and for all, she finally admitted: "It's him - it's Derek!"
EastEnders kicked off the mystery affair storyline in July, originally lining up five Walford residents as suspects before whittling them down.
Kat told Alfie about the affair in September without revealing her lover's identity, but the Moons only managed to get their marriage back on track for a short while.
In a recent interview with Digital Spy, Shane Richie - who plays Alfie - confirmed that his character will embark on a relationship with Roxy Mitchell in the aftermath of tonight's episode.
Speaking at the time, Richie added: "I think Rita Simons is now playing a different kind of Roxy, as the character is seeing somebody who she could spend the rest of her life with.
"At the same time, though, Roxy is feeling a little bit insecure because this relationship has happened very quickly. Roxy worries that Alfie might be on the rebound and could end up going back to Kat."
EastEnders continues tomorrow (December 21) at 8pm on BBC One. Get the Inside Soap magazine on your iPhone or iPad