Phantasy star online is a classic game that defined the Dreamcast console. One of the original MMORPGs, PSO was a pioneer in the genre and set the bar high for future games.
The basic plot of the game is that your people found a plant named Ragol, and when they sent down a ship named Pioneer 1 to investigate, it possibly crashed and became lost. The principal is looking for brave soldiers (which is you) from Pioneer 2 to investigate the crash and try to rescue any survivors. By completing side quests, you learn what happened on the plant piece by piece while discovering all of the different areas.
In the game there are three major classes. The hunter is best for close combat, the ranger is best for guns and long distance combat, and the force uses magic and other spells in a mainly supportive role. Each class has 4 different characters in it with slightly different attributes and skills. I usually played as the Ramar Ranger.
The online portion was a big part of this game, but the servers have been down for years, so it is now just a single player game. Even though the online part was taken away, there is still a lot to do in this game and you will get plenty of gameplay time.
Overall, this game is a blast, and if you haven't played it yet, go ahead and buy one!