Relations between Gibraltar and Spain crumbled in the summer of 2013, as the public was subjected to politically-motivated and incessant delays at the frontier between both countries, leading to stress and anxiety for everyone affected. The extraordinary events that led to the situation, as well as their repercussions, were featured in a special GBC documentary. 'The Invisible Frontier' focuses on the many different "frontiers" that exist between the towns on either side of the border; a frontier in its multiple facets and faces - historical, legal, political and human.The sequel, airing on Sunday, follows the same format. Politicians express their views on the continuing issues affecting the communities on both sides of the border, and also features local, UK and Spanish historians/academics on specific issues, such as constitutional matters, the Law of the Sea, self-determination etc. The programme also features some human stories on how people are directly affected by the Spanish authorities' actions at the border.