Dexter, Season 4

Debra:
I have something to tell you. And maybe I shouldn't. I don't know.

Dexter:
OK.

Debra:
I found another CI Dad was sleeping with. A woman named named Laura Moser.

Dexter:
[Voiceover: Harry was right. This is it.]

Debra:
Does that name mean anything to you?

Dexter:
Should it?

Debra:
It doesn't? Look, she died, but she left behind two little boys. And one of those little boys was you. Laura Moser is your biological mother.

Dexter:
[Voiceover: Don't say the rest. Don't know it.]

Debra:
The other little boy, your brother, was named Brian. Brian Moser - the Ice Truck Killer.