Grand Theft Auto: San Andreas/Missions/Key to her Heart

This mission is very simple. Your object is to start dating a employee at a competing casino. You'll drive to the Casino, wait for the girl to get in her car and follow her. First to a erotic toys shop where she'll buy a SM gear and reveal herself as a submissive, and you'll get to put on a catchy Gimp Suit. Then follow her home, kill her intended date(her "Master") then enter the house.

Basic Game
Millie becomes another girlfriend in the game and has to be romanced as per normal. After your progress with her reaches 40% or more she should call you to that she wants in on the plan and will give the swipecard required for the bank job in exchange for a cut. You can then collect the card from her house.

Note that when you are waiting for her boyfriend to show up, he will not appear on the radar. Look for the bald guy with a purple double-headed dildo. You need to dispatch him before he gets to Millie's front door.

If you are not inclined to romance Millie in the 'traditional' way, there are is a relatively easy (and amusing) way to get the card.

First of all, purchase the house in Prickle Pine so you have a save point close to her. The Gimp suit shows up in your wardrobe closet once you have completed Key to her Heart. Put on the Gimp suit and drive over to Millie's house. Note that she won't be there all the time (she works the graveyard shift at Caligula's), but if you show up in the afternoon she will be there. She will invite you in and the two of you will get busy (you see an external view of the house with the camera swinging around crazily). After the 'date' you will be back on the sidewalk with your default street clothes. Return to the Prickle Pine house, save the game, switch back into the Gimp suit and repeat.

Do this two or three times and you will win over her heart, and she will hand over the card.

Hot Coffee Mod
After killing the Croupier's master you have to pleasure her with a thorough spanking. Tap space just before the power bar reaches the maximum. If you manage to satisfy her the mission is successful, but you still need to make progress with her as a girlfriend to get the keycard.