It really isn't hard to follow.
Mary Ellis killed herself because she was being blackmailed by Mrs Huber, because Mrs Huber had discovered Mary Ellis' secret from her sister, who knew Mary Ellis when she was called Angela. That secret was that they had adopted Zach, who was then called Dana, from a druggie, and when the druggie came back for him they killed her, put her in the toy chest. They changed Dana's name to Zach and Paul told him he had a baby sister called Dana who died to cover it up.
Now the druggie whose baby they were looking after and who they killed, was the daughter of that old guy who is dying and wants to know what happened to his daughter before he dies. He's hired Mike, Susan's boyfriend, to find out what happened to her. Mike was going out with her at the time as well, so I reckon the final secret is gonna be that Zach's (Dana's) real dad is Mike.
Phew!