Kat gave birth to twins in 1984 when she was only 14 years old, but she only remembers the arrival of one of the children - her daughter Zoe Slater.
The truth was revealed to viewers in Friday evening's emotional episode (May 1), which saw Kat pay a visit to the convent where she remembered giving birth to Zoe.
Kat confided in a nun called Sister Ruth (June Whitfield) who had been at her side at the time, but once Kat left it became apparent that Sister Ruth knew one vital detail that she didn't.
Checking the 1984 birth records to verify her suspicions, Sister Ruth found an official note from Kat's previous visit which read: "February 1, 1984. Kathleen Slater, 14. Live births - 2. One girl and one boy."
As Sister Ruth only did her digging once the visit was over, Kat remains completely unaware of the life-changing revelation.
Show bosses are keeping tight-lipped about all further details, leaving fans to wonder whether the secret of Kat's son could take centre stage in EastEnders' upcoming Ireland-based spinoff featuring her and husband Alfie.
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Some fans are also speculating that Stacey Slater's brother Sean is the mystery man in question - could it be that Jean isn't his biological mother after all?
Maybe Sean is Kats's son!!! #EastEnders
— Laurs (@_LauraClarke_) May 1, 2015
Oh God. When was Sean born? #EastEnders
— Josh Allsopp (@JoshAllsopp) May 1, 2015
Sean's Kat's son isn't he? This is the only way I see this story going the way this episode is playing...#EastEnders
— Christina A. (@MissChrissy_92) May 1, 2015
So Sean must be the son?? #EastEnders
— Josh (@JoshAaronR) May 1, 2015
Friday's EastEnders episode also delved into the long-running mystery of Stacey's key, as it was revealed that Sean was the person who asked her to take care of it.
Although the character did not appear on screen, Sean spoke to Stacey in an online video call and reiterated that she should keep the key close by - even though she has no idea what it opens.
Confiding in Whitney Dean later, Stacey admitted: "He said it was my dad's. He just said to look after it - it's important."
EastEnders continues on Bank Holiday Monday at 8pm on BBC One.
