Cassie's rescue

Finding friends
You can't deal with Don on your own; firstly, you need some friends.

Don's little secret
Firstly, go to the building marked as the "Condo", and pay $2000 to rent one. After resting here, Lust Doll would meet their new neighbour, a girl called Fawn. From now on, you can visit Fawn when you want to talk with her, as long as you keep resting after each visit.

Keep talking with Fawn until she invites you in her room and leaves you alone (this event is repeatable as long as you don't progress further through this quest). Search the room until you find the membership card of some place called... "The Torture Rack"?..

The short way
Just ask Fawn about the card the next time you visit her. This would skip straight to the next part of the quest. Fawn would also offer you to become her subbe in "The Torture Rack" club, starting her own quest-line.

The long(er) way
Go to "The Torture Rack" (it's in the Slums), and try to learn more by asking a receptionist. Unfortunately, they would tell you nothing since they protect their members' privacy. You may sell the membership card, or try to bust Fawn exiting the club. Either way, she would confront you later and just tell what yes, she works at BDSM club, and would offer you to become her personal sub (starting her own quest-line).

The new ally
After learning about the BDSM club, you can tell Fawn about Cassie and Don, after which she would reveal what Don is actually one of her club's patrons.

After resting, visit Fawn again and discuss with her the possible ways to deal with Don. After talking about each possible option, you would come up with plan of actions.

Civil unrest
Without the police involvement, saving Cassie would be impossible. Visit the local Police Station and talk with the girl called Arlene.

Unfortunately, police has sort of... "agreement" with Don: as long as he keeps the Slums in check, the police wouldn't bother him. But it's in your power to end this alliance.

Visit the Slums again, and find the local homeless (to the south from the Don's hideout). One of them would step out and demand lots of money, calling himself the "leader of the crowd", but don't agree on this just yet: this guy is a scammer.

If you refuse him several times, he would leave, allowing you to continue, otherwise, he may be found in the Gentlemen's Club, but would only return the $500 you need to finish the quest; everything else was already wasted on strippers.

People who stay after you deal with the self-crowned "leader" would demand those $500 to organise everything. Unlike the first guy, they may be trusted. Rest and wait until they do everything, then talk with them again.

While you were sleeping, the homeless have spark a rebellion, spoiling Don's relationship with police and thus allowing Arlene to intervene. Go back to the police station and talk to Arlene – she will now agree to take Cassie into police protection if you rescue her.

From now on, you are technically ready to bust in, but you can make much easier by learning about Don's men's schedules. first opportunity arises immediately, since Fawn herself knows some of them as clients of her club. But there are two other girls who may help, too – Lin-Lin and Sammy.

My kung-fu is better than your kung-fu
Lin-Lin would only help if she gets payed first ($300), but she significantly increases chance of dealing with guards outside, or successfully stealing the key. Either option may be combined with asking her about guards' schedule, increasing your chances to pass skill checks once inside by additional 20% (adds up to your basic chance).

Hot bath
To receive the help from Sammy, you need to befriend her first. Keep paying her for bath services ($5) until she would offer you "special" for $40. After you accept it, she would become friendlier to you, and even offer you to spar together sometimes.

After talking with her again after receiving an invitation to spar together, she would offer her help with saving Cassie. She can't do much, but she would tell you more about guards schedule, allowing you to bypass some of them. This would add additional 20% to all skill-checks when in hideout; this is cumulative with bonuses from two other girls.

Getting inside
There are several ways to infiltrate Don's hideout, but all of them involve hard skill-checks.

To get it inside, you may steal the keys and try to sneak through, or you may deal with guards violently.

Skill checks are:
 * Attack guards; unsuccessful attack results in a Bad End.
 * Without Lin-Lin's help: 10+(2*Str)% chance.
 * With Lin-Lin's help: 50+(5*Str)% chance.
 * Steal door key: 50+(5*Spe)% chance.
 * You may only attempt to steal the key once. If you fail it, your only other option remains to deal with guards.
 * After stealing the key, you may temporarily retreat if you are not sure in yourself.
 * Allows you to climb the wall, bypassing guards entirely.
 * Seduce guards: (Ero 7+) chance.
 * Steal keys: 40+(3*Nim)% chance.
 * Attack them: 20+(2*Str)% chance.
 * Climb the wall (Strength 7+). Only becomes available after you obtain the key.

Finding Cassie
After you infiltrate Don's hideout, you can try to bust through, persuade guards to just leave, or sneak past. Failing to do either result in a Bad End.

Skill checks are:
 * Charge in:
 * Without any help: 10+(0.5*Str)% chance.
 * With the help of only one girl: 35+(0.5*Str)% chance.
 * With the help of two girls: 55+(0.5*Str)% chance.
 * With the help of all three girls: 75+(0.5*Str)% chance.
 * Sneak through:
 * Without any help: 20+(1*Nim)% chance.
 * With the help of only one girl: 40+(1*Nim)% chance.
 * With the help of two girls: 60+(1*Nim)% chance.
 * With the help of all three girls: 80+(1*Nim)% chance.
 * Negotiate:
 * Without any help: 0+(0.34*Spe)% chance.
 * With the help of only one girl: 20+(0.34*Spe)% chance.
 * With the help of two girls: 40+(0.34*Spe)% chance.
 * With the help of all three girls: 60+(0.34*Spe)% chance.

If you didn't recruit Arlene, your fate depends on wether you dealt with guards downstairs, or not.
 * If you sneaked past guards, they would ambush you from behind once you confront the Don (regardless of choice), resulting in Bad End, as you would be captured.
 * If you knocked up guards, you may choose wether to fight Don or try to negotiate with him. Again, either option results in his men catching up with you and attacking – but this time, you would be ready, initiating the battle with Don's guards. Either outcome would result in Bad End, but which one you receive depends on wether you win or lose:
 * Losing the battle leads to you being captured, resulting in the same Bad End as if you failed any other skill check while inside or were ambushed.
 * Winning opens the other choice (both of them would lead to Lust Doll and Cassie being forced to run from New Ark to save their lives):
 * Killing the Don results in police going after you.
 * Sparing the Don results in police and Don going after you.

If you did recruit Arlene, the quest continues.

Going out
If you either charged in or negotiated with guards, the quest would end immediately, with you going out without further resistance. Otherwise, the guards would try to block your way out, forcing you to deal with them again.

Skill checks are:
 * Charge in:
 * Without any help: 25+(1*Str)% chance.
 * With the help of only one girl: 45+(1*Str)% chance.
 * With the help of two girls: 65+(1*Str)% chance.
 * With the help of all three girls: 85+(1*Str)% chance.
 * Sneak through:
 * Without any help: 0+(2*Nim)% chance.
 * With the help of only one girl: 20+(2*Nim)% chance.
 * With the help of two girls: 40+(2*Nim)% chance.
 * With the help of all three girls: 60+(2*Nim)% chance.
 * Negotiate:
 * Without any help: 0+(0.34*Spe)% chance.
 * With the help of only one girl: 20+(0.34*Spe)% chance.
 * With the help of two girls: 40+(0.34*Spe)% chance.
 * With the help of all three girls: 60+(0.34*Spe)% chance.

Failing any of the sill checks would result in your capture and a Bad End.

Passing any of the last set of skill checks (or dealing with guards preemptively) allows you to evacuate Cassie from here, while Arlene steps in and... "deals" with the Don. In her own way.