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After completing "The Big One", the final in-game story mission will pop up at Franklin's mansion. Before attempting this game-changing mission, be sure to SAVE your game so that you can choose whichever mission you want and then retry it again if you want to do a different mission.
When Franklin approaches the waypoint in his dining room, Devin knocks on the door offering an ultimatum. From here, Franklin has only one of three choices:
Remember, once you choose one of the three options, the mission starts immediately! There's no turning back, so make sure you've accomplished all the things you wanted before doing the mission.