The games protagonist is Twinsen, a troubled Quetch who resides in an asylum due to his dreams depicting a coming apocalypse. After escaping the asylum, he returns home to his girlfriend, Zoe, only to be followed by some of Dr. Funfrocks clones. Dr. Funfrock is reveled to be the games antagonist, forcing the planets inhabitants to flee to the southern hemisphere to seek refuge from his teleporting clone army. Zoe has Twinsen hide from the guards and in doing so is captured and taken away. As Twinsen travel's across the various islands of Twinsun, it is soon revealed that his dreams are in fact a prophecy set fourth from an ancient being known as Sendell who lives in the planets core.
Long ago Sendell chose Twinsens ancestors to help protect the planet and is now using Twinsens dreams to send out a message in regards to the dangers of Dr. FunFrock. Twinsen then teams up with a rebel group to help gain access of one of Dr. FunFrocks teleporting devices so he may travel to Dr. FunFrocks fortress. In one of the fortresses holding cells, Twinsen finds Zoe locked away and tries to rescue her only to find that it was actually one of Dr. FunFrocks clones. Twinsen is captured leading Dr. FunFrock to believe that the events of the legendary prophecy will not come true. Dr. FunFrock then continues with his plans of drilling a hole in the center of Twinsun to gain access to Sendell and absorb her powers. Twinsen escapes the fortress, destroys it, and heads to the drill site. At the tunnel entrance, Dr. FunFrock awaits Twinsen's arrival as he holds Zoe captive. Dr. FunFrock then tries to negotiate with Twinsen for their safety in exchange for access to the tunnel. Twinsen and Dr. FunFrock engage in sword on sword combat that ends with Dr. FunFrock being pushed off a cliff. Twinsen and Zoe open the tunnel pathway and venture fourth into the core to meet Sendell, but soon are confronted by Dr. FunFrock who survived the fall. Twinsen defeats FunFrock in a sword battle and finally meet Sendell who thanks them and sends them back to the surface. The game ends with the planets inhabitants celebrating.
Relentless: Twinsen's Adventure takes place on the planet Twinsun. Unlike planets of our own solar system, Twinsun is suspended by two suns thus creating a polar region around the planets equator instead of its northern and southern points. This may also be the origin of the planets name.
Various species inhabit the planet, and the game centers mostly around four of them; the Quetchs who are human-like creatures, small sphere creatures known as Spheros, Rabbibunnies which resemble large rabbits, and Grobos which resemble elephants.
Twinsen - Protagonist and ancestor of the chosen protectors of Twinsun. Voiced by Dana Westberg.
Zoe - Twinsens girlfriend. Captured by Dr. FunFrocks clones. Voiced by Trish Kessler-Caffrey
Dr. FunFrock - Antagonist. Took control of Twinsun, his fortress is located on the northern hemisphere. Plans to drill a hole to the center of the planet to absorb Sendells powers. Voiced by Christian Erickson
Sendell - Ancient goddess that protects Twinsun and lives in the planets core. She first communicates with Twinsen through his dreams. Voiced by Trish Kesslar-Caffrey