"Sonic Tonic" is the fifteenth episode of the Sonic Underground television series. It first aired on 24 February 1999 in France and on 17 September 1999 in the United States. It was included in Volume 1 and Sonic Mega Mix.
The song performed in this episode is "The Fastest Thing Alive" (also known as "I Wish I Could Go Faster").
The episode begins in the Hanging Gardens, a place Queen Aleena built for the royal siblings. It then cuts to Robotnik cutting down the trees until finding the fabled Velocitree. At his mobile lab, he then proceeds to processing the tree, commenting it is the fastest growing thing on Mobius; with its essence, anyone can have sonic speed. The result is a bright yellow liquid, now called "Sonic Tonic". Dingo is a test subject for the mix. Sleet takes it after Dingo speeds away. Remarking on how the concoction may be dangerous, Sonic forbids his siblings from using it, leading them to remark behind his back that he needs to be "put in his place" and stop thinking that just because he's the fastest he's the leader of their team.
After putting a system in place, Sonic hangs around Robotropolis and is captured. However, after he is put in prison, Sleet and Dingo's feet grow to gigantic size, which leaves them unable to stop Sonic from escaping with the Sonic Tonic. When they have a blowout, Sonic leaves Sonia and Manic to fix the tire, which makes them use the Sonic Tonic. The same thing happens to them, but Sonic fixes it and the Hanging Gardens.
- Each being who used the Sonic Tonic had a different color when they sped away: Dingo (orange), Sleet (purple), Sonia (pink) and Manic (green).
- This episode has a very similar plot from the Adventures of Sonic the Hedgehog episode "Full Tilt Tails" and may have reused it for this episode.
- Before the episode aired on The Hub, on April 1, 2013, during April Fools Day Marathon on April 1st, after they saying "What's the day today?".