One Ring Challenge 2 is the fifth and final Challenge Act of Rooftop Run Act 1 in the console/PC version of Sonic Generations. It is unlocked after clearing every Stage in the Modern era and is playable only as Classic Sonic.
This Act takes place within a highly modified version of Rooftop Run Act 1, where there are stone boxes, Spike Traps, and additional enemies placed throughout. 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.
In this Challenge Act, the objective is to reach the Goal Plate at the end of the course.
The Act is similar to the normal version of Rooftop Run. The player begins on a section of aqueduct before being launched into the foreground, where they must travel to the left while avoiding Egg Fighters, Egg Launchers, and Spikes. The player will soon be launched by a springboard into the wine cellar area, and must avoid a gauntlet of enemies and spikes before reaching the Goal Plate at the left of the large room.