Planet Freedom

Planet Freedom is an alternate version of Mobius in an unnamed Zone. On Planet Freedom, its version of Dr. Robotnik kidnapped Princess Sara in order to force her to marry him. Sonic and Tails battled Dr. Robotnik and his robot, Metal Robotnik, briefly after their Mobius was altered to resemble this dimension by the efforts of Chaos Knuckles. The restoration of reality ended this incident and possibly erased it from the minds of those on Mobius. (StH: #101)

Background Information
Despite being a nod to the Planet Freedom from Sonic the Hedgehog: The Movie, this Planet Freedom's origins appear to be different. Like in the OVA version, Robotnik's hostage was a girl named Sara. However, unlike the OVA this Sara was referred to as "Princess Sara", whereas in the OVA Sara was the daughter of the President of South Island.