The Invincibility Challenge is the fifth and final Challenge Act of Chemical Plant Act 1 in the console/PC version of Sonic Generations. It is unlocked after clearing every Stage in the classic era and is playable only as Classic Sonic.
The player takes control of Classic Sonic in the Chemical Plant Act 1, where they are provided with no Rings at all. Many large stretches of the stage and platforms are also covered in spikes and enemies, making it impossible to traverse alone. As such, the player has to utilize the several constantly-regenerating monitors that offer invincibility power ups to the player in order to make it to the goal.