Set in 2090, the story of Front Mission: Online takes place on Huffman Island, a fictional Pacific Ocean island roughly the size of Oahu, created by volcanic activity south of Mexico's west coast in 1995. In 2002, the land mass was classified as an island, and was ceded to United Nations control. However, in 2020, the United States of the New Continent made a bid for control of the island after withdrawing from the United Nations. The Oceania Cooperative Union disputed this claim when the two superpowers colonized the island in 2065. The tensions heated up and eventually led to the 1st Huffman Conflict in 2070, with Huffman Island being divided into two halves at the end of the war. An uneasy peace was maintained until the Huffman Crisis in 2086, when a series of skirmishes across Huffman Island causes chaos on the island. Tensions flared up once again and eventually led to war when the O.C.U. is blamed for inciting the Larcus Incident on June 3, 2090.
The plot of Front Mission: Online takes players from the beginning of the 2nd Huffman Conflict to its end in 2091. Unlike Front Mission, the game's plot covers both the minor and major events that occur during the war, such as the war-ending Operation Eagle Tears (coincidentally, this was also the final military operation, taking place from May 27 to May 31, 2008). Online revolves around four military units that play a major role in the war: the Chasm Owls (O.C.U.), Iron Hearts (O.C.U.), Proud Eagles (U.S.N.), and the 17th Mobile Company (U.S.N.). These four military units influence how the war turns out, from the early stages when the U.S.N. military was winning the war, to the later stages when the O.C.U. military begins using mercenary outfits. Throughout the game's service, players could fight alongside or against characters and groups from Front Mission. Additionally, characters from the other Front Mission entries made appearances in several missions or military operations as either allies or enemies.
These are the recurring characters whose first appearance was in Front Mission: Online. The video game takes place from 2090 to 2091.
Ellen Taylor is an O.C.U. officer in the employ of the O.C.U. reconnaissance unit, Chasm Owls. Ellen is deployed to the Mail River border on Huffman Island, and is engaged in espionage and reconnaissance activities against the U.S.N. Ellen also makes an appearance in Front Mission 2089: Border of Madness.
Ernest Miller is a U.S.N. officer involved with the U.S.N. Border Patrol Force. During the outbreak of the 2nd Huffman Conflict, Ernest was assigned to monitoring O.C.U. movements by the Mail River border. Ernest also makes an appearance in Front Mission First.
Yuji Kinoshita is an O.C.U. mechanic working with both the O.C.U. military and its hired mercenaries. Yuji is assigned a post on Huffman Island in 2089 to train new mercenary recruits on wanzer piloting and following O.C.U. military procedures. Yuji also makes an appearance in Front Mission 2089: Border of Madness.