- "A Missile Launch Base has fallen into the wrong hands. Storm the base, clear it out, stop the warhead from launching, and save the day."
- — Description
The Doomsday Scenario is the final part of The Doomsday Heist. Players are in a race against time as they infiltrate a secret doomsday facility that was sealed away for decades within Mount Chiliad before Avon Hertz and Cliffford can launch a nuclear warhead to destroy all of humanity.
Players are tasked to go to a lot in Grapeseed, where the team can choose from either the Khanjali tank or the Barrage ATV to use in the team's assault of the missile base. The team can only choose one vehicle, with the other being sold to gain an extra amount of cash at the end. Once the team's vehicle is chosen and every player is on board, the team must drive to the tunnel at the Chiliad Mountain State Wilderness, where the secret entrance to the missile base will open behind a set of electrical boxes.
The team must use their vehicle to drive through the tunnel, fighting off Avon's men. After reaching the end, the vehicle will be disabled by spikes on the road, where four Juggernauts will ambush the team. When they are dealt with, the team must move through the base on foot, dealing with more of Avon's men as Cliffford taunts the team. Eventually, there will be a fork in the tunnel, which diverges off into two different areas. In both of these tunnels are a combined total of five supply crates that the team must destroy. Once all five are destroyed, the paths will reconnect. After dealing with more of Avon's men and two more Juggernauts, Cliffford will begin the launch protocol for the warhead. The team will encounter a locked door, which is opened by Cliffford after he is tricked into opening it by Lester.
The team must now go around a large square room and hack into four terminals, which will cripple Cliffford and abort the launch of the warhead. Once all four are done, the team can go to the top floor, where four Orbital Cannon weapon systems are available to use by all four players. The team must use the cannons to destroy four mobile cloud servers around the state, which will destroy Cliffford once all four are down. Afterwards, Lester will locate Avon trying to escape. The team must press on through the facility, fighting their way to the silo itself, where Avon escapes with the use of a Thruster jetpack. The team then takes their own Thrusters and gives chase, ultimately shooting Avon down and killing him whilst dealing with several attack helicopters.
With the deed done, the team regroups at the Galileo Observatory, where Mrs. Rackman will thank the team for their efforts, awarding them with a large sum of money before leaving with Agent 14. Lester then thanks the team as well before parting ways, ending the heist.
- Avon Hertz - Killed by the GTA Online Protagonists for attempting to launch a nuclear warhead and for attempting to escape.
- Cliffford - Destroyed by the GTA Online Protagonists via the Orbital Cannon for attempting to launch a nuclear warhead.
- As stated by Lester, though the Khanjali is much more tougher for the initial assault on the facility, it will pay more at the end when the team sells it afterwards.
- It is recommended to have Snacks and Body Armor on standby, as Avon's men, notably the Juggernauts, are heavily armed and can easily kill a player if they aren't well equipped.
- If the player takes the Khanjali and is able to get to the end of the main tunnel within the mountain, its main turret can be used to easily used to dispose of the four Juggernaut waiting for the player at the end.
- If the player dies before they reach the control room, they will appear at the end of the tunnel, where the Khanjali can be used again to destroy the initial Juggernauts.
- If players opt to use the Barrage instead, driving it in reverse while in the tunnel will make the most out of its Grenade Launcher.
- Do note that if the mission is done as an Organization, the bodyguards will gain the ability to heal to full while in cover, which will conserve snacks and body armor. This does not apply to the CEO, nor does it apply to any players completing the heist finale as a Motorcycle Club, however.
- The player who is hacking the servers is immortal during the hack, although they will be unable to respond for about a second after leaving the hack, during which they can be shot and killed. It is recommended that the player(s) who aren't hacking clear the area around the hacker just before they come out of the program.