Ecco the Dolphin: Defender of the Future was actually the fourth game in the Ecco the Dolphin series though it was more a remake than a sequel. It was developed by Sega and released for the Dreamcast in 2000. The main advancement in regards to this game was the move from 2D to 3D. Previous games in the Ecco series were released for the Sega Genesis and Sega CD.
The game encompasses a large span of time and is set in the distant future. Dolphins are as intelligent as humans and are at different point allies or enemies of human. As Ecco, the player is always attempting to unite them and save the earth from various threats, the biggest of which is an alien invasion.