I would say out of the hundreds of games I have played, the call of duty franchise has the most insane story lines. This is solely because without putting in the time and effort into the game you would not realize that you are actually participating in a plot with each character having a specific role in the story. Without having prior knowledge of the background of each game, you would not be able to access the Easter Eggs (they are a series of challenges to unlock alternative endings to games) within the games and therefore unlock the story line.
For example, Call of Duty Zombies section of the franchise commenced in their beginning zombies game with Call of Duty: World at War with a few zombie maps but nothing that really started off the plot. In Call of Duty: Black Ops, the main plot started with the map, Moon. The player was chosen to play as one of the 1.0 crew that consisted of Corporal “Tank” Dempsey, Nikolai Belinski, Takeo Masaki, and the German scientist Edward Richtofen. You would play to complete a series of challenges on the base that was established on Earth’s Moon for the Easter Egg that would reveal a cut scene where Richtofen’s daughter Samantha is stuck in a device that would allow her send and control all the zombies on Earth. Later in the challenges, 3 rockets would be sent to Earth and the whole planet would be completely annihilated.
In Call of Duty: Black Ops 2, you would play as a new set of wastelanders on the map TranZit, where the map takes you to the destroyed Earth and you are trying to survive by traveling on a bus to different part sof the map. You would complete the Easter Egg Challenges but mostly this game was used to prove that Earth was completely destroyed and it wouldn’t be able to be repaired without a lot of work. On the 2nd map on the Black Ops 2 game called Origins, the Easter Egg challenges would end in the previous universe being completely eliminated and a new one being opened in its place and those in the 1.0 crew being reincarnated as warriors in a group called Ultimis. In the Call of Duty: Black Ops 3 map called Revelation, it shows the players trying to set the Universe right from all the evils and completely get rid of the zombies. As well as Samantha being released as the controller of the zombies (that is shown by the different colors in the eyes of the zombies).
The story is more complex but this is the best description that I think someone who does not necessarily play the franchise can understand.