At the start of this mission, you'll find Ray Boccino from GTAIV's main story talking with Billy Grey, leader of The Lost. Johnny is introduced to Ray, and immediately afterwards, Billy decides it's time to inflict pain on The Angels of Death for supposedly killing Jason Michaels, though he said in a previous mission that the person to blame was an "Eastern European" (Niko Bellic). After the cutscene, you'll be on your bike in the alleyway to the left of the Lost MC clubhouse, along with a few of your men and Billy. After driving a little while, Billy randomly declares a race, the winner reaching the AOD clubhouse first (naturally this has to be you). Fortunately, there's a lot of straightaways where you can gain huge momentum and easily beat the competition.
Once everyone's on-scene, Billy will hand Johnny a grenade launcher and tell him to flush out the AOD members inside. Firing a single grenade into the open window of the clubhouse suffices. This flushes a bunch of AOD members into the open where they engage in a firefight with the Lost MC. Once all is clear outside, you and Billy will head inside to clear out the remaining forces. Upon reaching the basement, a large heroin stash (referenced as being $2 million in value) is discovered. It becomes apparent to Johnny that all of this was a rouse by Billy - the killing of Jason was indeed by an unrelated Eastern European (as we know from the early main story), and this was instead about scoring the heroin stash. Regardless, what's done is done, and you're ordered back to your bike. The LCPD will arrive on scene, but dodging them should be easy (no wanted level involved), and once you've evaded them, the mission is complete.