Summoner's Lineage takes place in the same world as Tales of Phantasia, in the year 4765 by the Aselia Calendar (411 years after ToP's future). As a young summoning apprentice, Fulein K. Lester and his sprite-like companion Macaron are studying the principles of calling upon magical beasts into the physical world. However, their exploits are cut short when a significant number of monsters and other beasts begin to appear within their world, threatening the stability of their long-peaceful nation. It is decided that in order to combat these creatures, the legendary Heroes of Time (the name given to the main characters of Tales of Phantasia after their adventures across several centuries in the previous title) would have to be called upon. It is then that Cress, Claus, Mint, Suzu, Arche, and Chester make their appearance, and with their combined skills, along with the help of some new faces, they will hopefully rid the world of this new crisis.
Source: Wikipedia, "http://en.wikipedia.org/wiki/Tales_of_the_World:_Summoner%27s_Lineage," available under the CC-BY-SA License.