And, as our preview pictures reveal, it’s not the baby’s real father – and Martin’s best mate – Kush Kazemi (Davood Ghadami) who tells Martin the truth.
The bombshell is dropped on Martin through a letter that it seems Stacey never meant him to see.
Signed, sealed and accidentally delivered: the letter that tells Martin he's not Arthur's father
“Martin is accidentally given a letter from Stacey after a nurse at the hospital where Stacey’s being treated [for her mental illness] assumes he dropped it,” reveals our EastEnders insider.
“When he gets home he sits down with the baby he adores and decides to read the letter.”
And that’s when his heart breaks.
“The letter tells Martin that Arthur isn’t his son,” adds our source. “And it reveals that Kush is.”
That’s the end of that friendship, then.
Martin looks at Arthur, struggling to believe the baby is not his son
Martin sits alone with Arthur, in tears. This baby has felt like his son since before he was born. How can he not be his father?
We can’t reveal what happens next but we can confirm that, as you would expect, Martin doesn’t take this news well.
Stacey (Lacey Turner) has been slowly recovering in hospital from her postpartum psychosis but Martin’s reaction to this shattering revelation could have tragic consequences.
This episode airs on BBC1, Monday, February 22.
EastEnders continues on BBC1, Monday, February 15, 8.00pm.