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.
This Challenge Act takes place within a highly modified version of 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 large number of constantly-regenerating Item Boxes that offer Invincible power-ups to the player in order to safely make it to the goal.
The player begins deep inside the Chemical Plant, near a waterfall. Before them lies an Invincibility monitor, which they must utilize in order to safely cross the pit of spikes that would otherwise block their path. After continuing onward, they will find other platforms covered by spikes, and additional Invincibility Monitors that they must break in order to survive the Act's hazards and get to the Goal Plate.