Wolfpack has no backstory and are ignorant of pretty much everything and anything going on. They don't seem to know much about Umbrella, the outbreak, the B.O.W.'s, their missions etc. There is no ongoing story between them. They're simply contracted to do missions with the 'goal' being if they want to be evacuated, they need to complete all missions. That is it.

They're told to escort Hunk to Birkin's lab to get the G-Sample. Birking Injects himself with the g-virus and comes after them. Hunk lets Wolfpack escape while he goes and fights Birkin alone since he lost the G-sample and needs to go back for it. They're then told to destroy evidence at City Hall against Umbrella where they meet Nicholai and tell him he's on his own and they won't help him. They're sent to the Hospital to get EMPS but Nicholai blows the hospital up and crashes the plane dropping the EMPs. Nicholai then lays traps for them to try and kill them, explaining they're in his way and his 'client' has their reasons. Wolfpack tries to take him out but ultimately move on and forget about him. Follow that by shutting off the power in Raccoon City with the EMPs.

Then they're told to collect a Nemesis Parasite from a Tyrant and implant it in the Nemesis to reprogram him, which results in them having to fight Mr. X and then they have to fight and immobilize Nemesis who simply wields a RailGun and walks around slowly doing nothing more than shooting it and smacking you with it if you get close. Upon being reprogrammed he simply drops the RailGun and picks up the rocket launcher, says "STARS" and moves on. Following that, they're told to go to the RPD and recover evidence Chief Irons has collected against Umbrella to blackmail them and eliminate all RPD cops. Hunters and Mr. X's are dropped into the city and Wolfpack is forced to deal with them and fight two Mr. X Tyrants at the same time. They notice Leon & Claire arrive and get seperated in the cop car and are sent to kill him (They never seem to care about Claire) but he gets away and Umbrella decides to leave U.S.S. to fend for themselves and screw them out of Evac.

Off-screen U.S.S. fends for themselves for awhile until Umbrella decides they need them again to take care of Ada who has infiltrated their labs. They promise Evac if Wolfpack completes the mission. They find Ada in the labs and do battle with her but she gets away. Wolfpack finds out Leon is working with Ada and is in the Lab as well and they witness Ada die in Leon's hands after Leon knocks Mr. X into the lava.

Wolfpack goes after Leon and Tyrant pops out of the lava and hops on the elevator to fight them then gets knocked off and is never seen again. Wolfpack fails in capturing Leon. Leon, Claire and Sherry escape the lab and are under the protection of the Spec Ops (just the generics, not the ones from the trailer) while waiting to be evacuated at the Train Yard. Umbrella orders Leon to be killed because he knows too much and Sherry captured because all samples of G were lost with the lab and Sherry was infected with G but now has anti-bodies in her blood which Umbrella wants or else no evac will come for Wolfpack.

Leon & Claire with Spec Ops make a stand against Wolfpack and Wolfpack moves in and captures them. You're then given the choice to either Excute Leon or Defend him. Two of your team choose one and the other two choose the other option.

-Killing Leon

If you choose to kill Leon, you simply have to kill the other two members of your team who defected and then Kill Leon. The ending plays with Leon dead and Claire begging them to stop and they off screen shoot and kill Claire then Take Sherry.

The End



-Defend Leon

If you choose to defend Leon, you simply have to kill the other two members of your team who have chosen to complete the assigned task. The ending plays with Leon questioning why they didn't kill him and your char explaining that Umbrella screwed them over. Umbrella demands Leon be killed and your char says only if their pay is tripled and they get evac. Umbrella declines to negotiate and your chars let Leon, Claire and Sherry go, vowing to make Umbrella pay.

The End

So instead of taking the train out of Raccoon City they now just take the elevator back up to the train yard and the lab explodes beneath them presumably? Then the final battle essentially takes place at the end of RE2?
I knew that train yard scene would never fit as soon as I saw it!

Any more hints on Ada's employers?

I'm guessing now that Nicholai's fate will be revealed in the Spec Ops DLC?

You only encounter Jill in their campaign, not the USS?