Samus is called by the Galactic Federation to investigate the planet of Aether after the Federation lost contact with the Marines who had crash landed on the planet after encountering Space Pirates.
While looking for the Marines near Aether, Samus' ship is damaged by severe lightning storms from the planet. Said storms have caused electromagnetic interference that prevented the Marines from communicating with the Federation. Samus finds the troops dead and surrounded by hive creatures called Splinters. The deceased Marines suddenly rise and attack her, apparently possessed, and she fights them off. Samus then encounters her evil doppelgänger, Dark Samus, for the first time, and after a small skirmish Dark Samus jumps through a portal. Samus decides to follow her through it and ends up on Dark Aether, a vile trans-dimensional duplicate of Aether, where she is attacked by a group of dark creatures called Ing, who capture Samus and after stealing the weapons from her suit, throw her back through the portal.
Upon returning to Aether, Samus learns that the Marines were attacked and killed by Ing-possessed Splinters, and decides to enter a nearby alien temple structure to look for clues. When she reaches the structure, she meets U-Mos, the last remaining sentinel of the Luminoth, an alien race that have fought against the Ing for decades. They are now on the verge of defeat. He tells Samus that after a meteor struck Aether, the impact was so devastating, it created "Dark Aether", from which the Ing spawned. He also tells Samus that the Ing have taken virtually all of the Light of Aether, the entire collective planetary energy for Aether that keeps the planet stable, and begs her to retrieve it, for if either world gains control over all of this energy, the other will perish. To reclaim the parts of Aether's energy taken by the Ing, she makes use of an energy transfer module.
Samus goes to 3 regions—the Agon Wastes, a parched, rocky, desert wasteland region; Torvus Bog, a drenched swamp area that houses a partially submerged hydrosubstation; and the Sanctuary Fortress, a highly advanced cliffside fortress built by the Luminoth filled with corrupted robots that serves as the Ing hive in Dark Aether—to retrieve the Light of Aether and return it to the Luminoth temples. Samus fights Space Pirates, Dark Samus, and monstrous Ing guardians on her mission. After Samus retrieves 3 pieces of the Light of Aether, she collects 9 Sky Temple keys and enters the Ing's Sky Temple to facee the Emperor Ing, the strongest Ing who guards the remaining Light of Aether. Samus defeats the creature and retrieves the last remaining energy, causing Dark Aether to become critically unstable and begin to collapse; however, her path out of the temple's gateway is blocked by a horribly altered and unstable Dark Samus. After defeating her foe in the final battle, Samus is surrounded by a group of Warrior Ing desperate to save their world and their lives; she escapes to Aether through a newly revealed portal just before Dark Aether and the Ing disappear forever.
Returning to U-Mos, Samus finds that the Luminoth were in a state of hibernation but have now awakened. After a brief celebration, Samus leaves Aether in her repaired gunship.
Post-Credit Scene: Dark Samus is shown reforming herself above Aether.
Metroid Prime 2: Echoes was re-released as part of Metroid Prime: Trilogy which adds features first implemented in Metroid Prime: Corruption such as motion controls and updated graphics.
F.A.Q.
Reward for Collecting 75% to 99% of Upgrades: collecting 75% to 99% of all upgrades results in the ending scene extending to show Samus removing her Varia Suit to show her Zero Suit before boarding her gunship.
Reward for Collecting 100% of Upgrades: collecting all upgrades (100%) unlocks the Post-Credit scene after the Mission Final screen.