The New York Supplementary Education Institute was a front for a Virtor plot to kidnap humans to be used as slaves on their home world. It ostensibly offered classes, but anyone signing up was taken to the Virtor home world.
The Second Doctor and Edward Grainger were both investigating the Institute separately but joined forces. They stormed the place and rescued Jamie McCrimmon, who had gone undercover to join a class. The Doctor was taken to the Virtor homeworld, where he led an slave revolt, and he rescued all the slaves before closing the portal from the Institute to the Virtor home world. (PROSE: The Lost)