Action/Reaction

Action/Reaction is a mission in GTA IV DLC The Lost and Damned. It is given by Billy Grey to Johnny Klebitz.

Overview
Johnny Klebitz is introduced by Billy to Ray Boccino. After Ray leaves, Billy wants to pay someone a visit, the The Angels of Death.

Description
At the start of this mission, you'll find Ray Boccino from GTA IV'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". After the cutscene, you'll be on your bike in the alleyway to the left of the Lost MC clubhouse, along with Terry, Jim Fitzgerald, Clay, Brian and Billy. After driving a little while, Billy randomly declares a race, the winner reaching the AOD clubhouse first. 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 ruse by Billy - the killing of Jason was indeed by an unrelated Eastern European 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 and once you've evaded them, the mission is complete.

Triva

 * This mission is based on the real-life action of The Hells Angels shooting a grenade into a clubhouse of the Bandito's.
 * When Johnny wins the race, he will mention that he's still Billy's "loyal servant". However, soon after these events, he starts to develop doubt feelings for how Billy leads the gang.