Not necessarily a story, but there is some history to be sure. The choice of Samaritan in this the parable is directly linked to the general message of the story explained in the last verses and the two characters that denied help to an injured man. There is also a good deal of negative stereotypization that resulted in the choice of Samaritans for the discussed purpose.

Samaritans, named after the city of Samaria, one of the capitals of the Northern Israel Kingdom, started to become a distinct cultural and ethnic group after the conquest of Israel by the Assyrian ruler Tiglat-Pileser III, his son Salmanasar V and his successor Sargon II who invaded the territory in the years 732-722 BCE. Conquered people often rebelled, so the rulers eventually ordered deportation of the large part of local population deeper into Assyria and colonization of Northern Israel by Assyrians throughout the 8th and 7th centuries BCE. Similar treatment of the inhabitants of Judea by Nebuchadnezzar II who transferred substantial part of the wealth of Judean Jews to Samarians in 6th century BCE has further exacerbated the rift between Samarians and Jews. The attempts to reconcile these two groups ended in failure, with both sides treating the other as apostates. In mid-4th century, Samaritans have built their own centre of cult on the Mount Gerazim.

The rift widened during and after the Maccabean Uprising in the 160s BCE, when the Jews rebelled against Seleucid rule after Antioch IV ordered the Jerusalem Temple to be changed into a temple to Zeus. Treating it as an act of sacrilege and an attempt to eradicate Judaism, Jews openly opposed the ruling Seleucids and thanks to series of military victories and Roman mediation, they managed to effectively achieve strong position as Syrian fief and later won an independence for Judea. During that conflict, Samaritans sided with Seleucids what later resulted in persecution by victorious Jews, culminating with the invasion of Samaria by John I Hyrcanus and destruction of the Samaritan temple on Mount Gerazim in 129 BCE. The hostilities continued throughout the Roman period, when the Jews and Samaritans were denouncing each other as the disloyal subjects and potential conspirators against Roman rule while not supporting each other in actual rebellions. These antagonisms were to some extent utilized by Romans who e.g. rebuilt the Gerazim temple after the Bar Kochba rebellion.

Thus, in the popular optics of the average Jewish inhabitant of Judea in 1st century CE, Samaritans formed a separate nation that has been more or less conflicted with Jews for centuries and although their religious and cultural tenets were similar and derived from a common source, they were also distinct to the point that could not have been considered a followers of the same religion. Using modern concepts, Samaritans were nearby foreigners from the country that was more hostile than friendly. Gospel of John (J 4:7-9) tells a story of Jesus asking Samaritan woman for water, to what she expresses surprise, as Jews were very unlikely to mingle or even speak with Samaritans due to their animosities.

And so we come to the actual parable, where the injured, robbed man (presumed to a Jew, as he was going from Jerusalem to Jericho) is ignored by a priest (kohen) and a Levite, i.e. people holding a important position within Jewish communities and usually held in a very high esteem as the religious leaders and officials. And then, he is helped by a Samaritan, a man who does not share his ethnic nor religious background but also is a member of a people that were known to actively work to detriment of the community of the victim of an assault. In other words, a Samaritan had no interest in helping someone who was a foreigner, member of a hostile nation and possibly an enemy. Thus, this parable is interpreted as the call to treat everyone according to their actual deeds, and not the national, political or religious affiliation. This is mirrored by another passage from the same gospel (Lk 17: 11-19) speaks of Jesus healing 10 lepers, out of whom only one returns to thanks him, and this man is said to be a Samaritan. In both cases, the examples more or less show that a follower of Christ should not be prejudiced and treat others for what they are actually represent.

So, to sum it up, the Samaritan was selected because at the time when the gospels were written, he was a shorthand for maybe not necessarily an enemy, but definitely a hostile foreigner or one of the last people a Jew could have expected help from, especially after being denied one by members of their own community. A good equivalent would be e.g. a Loyalist helping a Patriot during American Revolution or a Protestant helping a Catholic during the Thirty Years War.