Tails: Toxic Skies is the first 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.
In this Challenge Act, Classic Sonic runs through Chemical Plant Act 1, but he is greeted by rows of spikes that are too long to jump over and long expanses of sky that he cannot cross alone. In these instances, marked by a circular blue sign with a yellow exclamation point, the player must press / to summon Classic Tails. Tails then appears directly above Sonic, who must jump to grab on to Tails' hands. In order to fly left or right, the player must move the Left Control Stick left or right. The player must repeated tap / to fly up, or refrain from pressing it to slowly descend. Once Sonic's feet touch the ground (or if the player presses /), he lets go of the fox's hands and Tails disappears in a puff of smoke.
Halfway through the level, there is a vast gap that Sonic and Tails must cross to reach the goal. Sonic must fly across with Tails, avoiding balls of spikes and lasers in the way. There are also green rings that give Tails a short boost should be pass through them. Once Sonic passes the Goal Post, the mission is completed, and the player is ranked according to his/her score.
- In this Challenge Act, Classic Tails can collect Rings and use Propeller Flying indefinitely.
- This is the second and final Challenge Act that Tails appears in.
- This Challenge Act is the only place in Sonic Generations where the Mechanical Rings appear.
- When talking to Classic Tails after finishing this Challenge Act, Tails will say "It was really fun to go on an adventure with you! We should do it again sometime!".