During another visit to Haven City to deal with another conflict involving a minor Metal Head invasion, the Lodgers end up discovering that the invasion was caused by a new enemy. And not just any enemy, but the return of the thought-to-be-dead Errol, who survived the events of Jak 3, and has returned for revenge, this time having a partner in the form of Kagen, a former test subject for Baron Praxis' Dark Warrior Program, and is evenly-matched in skill to Jak. Though he was unstable in both emotion and power since he had too much Dark Eco in his system, and his old pure heart is the only thing keeping him alive from such an ailment. Kagen moved onto being a bounty hunter since his escape from Praxis' experiments. After a fight against the unstable Dark Warrior, Jak ends up requiring help from the Lodgers. Especially when the enemy escalates when they have located an ancient Eco Mine built by Mar long ago, giving them access to a lot of power and weaponry. The heroes eventually learn about Kagen's past from an old childhood friend of his, Cameryn, who was banished to Spargas for having a family against Praxis, and she aims to help her friend, and discovers that he has a secret crush on her. So with the Metal Heads and Errol growing stronger in power thanks to their unlimited supply of Eco, Jak's Crew and the Lodgers must find a way to destroy the mine, stop Errol for good, and bring Kagen to the light.
(Another climax theme)
(Daxter sings this at the end)