The woman at the beginning killed herself because she was being blackmailed by a neighbor who found out she'd bought a drug addicts baby on her. She thought the neighbor knew that she had killed the baby's mother when the mother had come back to reclaim the baby a couple years after.

The "mystery cop dude" (Mike) is the ex-boyfriend of the mother that was murdered. He was hired by that mother's father to find out who killed his daughter and to kill them. When Mike finds out who killed her, he takes Paul (the suicidal woman's husband) out to kill him, finds out why she killed Dierdra (the baby's mother), and changes his mind, realizing the boy is really his son.

Any more questions? I've seen every episode.