Worldwalker

Due to some force or trait, you have become familiar with travel between the planes of existence. As a result, you can use the Plane-Shift spell for free, once per week. Work with your DM to figure out how your character came upon this feat. Are they a part of a race which has the ability to worldwalk naturally? Was it the result of some experiment gone horribly wrong? Do they inherit the trait from their family?