Asuka wants you to find her kidnapped niece... You know, the one you just kidnapped on the last mission. She says you'll be rewarded for doing it.
Get Her Back
Asuka has given you $20,000 which is the initial part of the random money. You need to deliver it to the meet location at the North of the island.
Grab a vehicle and make your way North. Park in the mission marker when you get there and a guy will appear and tell you to leave the money at the drop off. Walk into the circle and he'll basically thank you for the money and run off.
You'll then have another delivery to make further North, so head there now. As you arrive you'll see that she's in a bus which is guarded by four guards. Mike suggests heading in through the back, so run at the wall and press A+B together to jump over it. As you approach the bus you'll discover you need a key, so run around killing the guards until you get it. Then you can get in the bus.
You need to make your way back South in the bus, without taking too much damage. Be careful not to turn too sharply because big vehicles tend to flip over very easily. Watch out for the vehicles chasing after you, but if you keep driving pretty quick you should be out of their range after not too long.
Once you reach Asuka's place she'll offer to whip you in her bedroom to say thanks, but Mike says he'd rather keep his skin intact.