"Flying Fortress" is the twenty-fifth episode of the Sonic Underground television series and the first of three episodes in the Chaos Emerald Crisis mini-series. It first aired on 31 March 1999 in France and on 1 October 1999 in the United States. It was included in Volume 2 and Secrets of the Chaos Emerald.
The song performed in this episode is "No One Is An Island".
What is supposed to be a vacation for Sonic, Manic and Sonia turns into a battle to save Mobius when Robotnik attacks with his newest weapon - the Flying Fortress. Sonic investigates the ship thinking the weapon can be destroyed by removing its reactor, however his efforts fail and he goes to Knuckles for help, because he believes the weapon is powered by the Chaos Emerald. Knuckles agrees to aid him suppressing the fear that the Floating Island will be left unprotected. Knuckles and Sonic locate power source only to have it taken by Sleet and Dingo before the Hedgehogs can reach it.