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The following is a brief description of the story in the M Tunnel. More information on the story can be found here.
After completing the M-Tunnel, the cutscene 'Insanity' will begin. The Duplicator is watching the Runner's attempts to reach a tunnel in the distance. She had jumped multiple times to reach it, but fell short every time. The Duplicator asks the Runner if she knows what they say about going insane, because he thinks that she is doing the same thing while expecting a different outcome. The Runner responds that she is not doing the same thing before tripping off the edge. When she respawns, the Runner notices that the Duplicator drew on her map. He tells the Runner that it is a picture of her to "commemorate this moment" so they'll remember when she went crazy. The Runner is surprised, but she ignores it and jumps again, this time successfully reaching the next tunnel. After the cutscene is finished, the player will start The River, part 1 with whichever character they used last. However, next time the map is viewed, the player can see that a badly drawn image of the Runner sticking her tongue out had been drawn near the gap between M-5 and The River, part 1 (F-1).