This is gonna be a long one! To rescue Cassie, you'll need help from Fawn (neighbor in the condo in New Ark), Arlene (psycho cop) and people in the slums. Figuring out the Don's men schedules from Fawn (assuming you've done her story arc enough to know she's a domme), Sammy, and Lin Lin helps but isn't needed. LD and Fawn will come up with a few ideas but nothing concrete. Talking to Arlene says the cops can't do anything so long as the slums stay quiet. That's a hint, to go to the slums, the area where you can rest. Talk to the "leader", he'll mention agreeing to do it for a given amount. Ignore those amounts until it gets to 500. You'll have to wait a day after agreeing, and see that the people raised hell.
Talking to Arlene says they've got their hands full, implying you're responsible. Go to Fawn, talk about rescuing Cassie, and Arlene shows up. They work out a plan, then you can go and rescue Cassie. You'll need strength at least 7 and nimble pretty high, to avoid being seen. If you successfully rescue her, you'll be shown an interesting scene between Arlene and the Don, then Arlene goes to administer a beat down (which you don't get it see). Arlene, nude except for her cop hat, shows up and says that the Don will agree do leave Cassie alone.