One Ring Challenge is the first Challenge Act of Chemical Plant Act 2 in the console/PC version of Sonic Generations. It is unlocked after clearing every Stage in the classic era and is playable only as Modern Sonic.
The Act takes place in a slightly modified version of Chemical Plant Act 1. The stage's layout has been modified by metal boxes, and additional enemies and hazards have been placed in the player's path. The player is given only one Ring at the beginning of the stage, with there being no further rings up ahead in the rest of the stage.
The player begins in a section just before the waterslide in the normal version of the Act. They must travel across platforms full of enemies and continue onward to the waterfall. After skydiving and running into the next section, the player must complete one final platforming section before reaching the Goal Ring above.