Metroid Prime 2: Echoes is set within the Metroid series, and is a direct sequel to Metroid Prime. Like its predecessor, it was a Nintendo GameCube first-person adventure game. The story is completely different, with new worlds and variations on Metroid Prime's enemies and weapons. The graphics have been upgraded and many things have been remodeled by Retro Studios.
A Galactic Federation team has been attacked on the planet Aether, located in the Dasha region. They were following a Space Pirate frigate, as in Metroid Prime, but their ship was damaged in descent to the planet. They split into two teams, one to defend the ship and send for backup, and one to investigate the Pirate operations. Unfortunately, they soon encounter hostile indigenous creatures. However, they seem different from the previous creatures encountered, which mainly went on with their life and ignored the troopers.
Samus Aran receives their distress beacon and quickly responds. On her arrival she discovers that the second team of troopers has been killed. She is able to recover enough of their logs to know that they were overrun by the hostile creatures. She also finds that there is someone with a strong resemblance to herself, though she seems to have a link to Phazon.
The planet is not peaceful. Once a world of light inhabited by the peaceful and enlightened Luminoth, Aether is now torn between the world of light and a mirror image world of darkness. The new world, inhabited by dark creatures known as the Ing, was created when Aether was struck by a meteorite, not unlike Tallon IV. The resulting impact, coupled with the fact that the Luminoth harness the local dimensional energy, created a pocket universe containing only Dark Aether, and the resulting imbalance caused by two planets attempting to occupy the same point in space/time created a dimensional game of tug-of-war.
As it turns out, the Ing have managed to steal the energy of Aether (which the Luminoth had embodied), the only thing keeping Aether on the winning side. Now only one of the four energy temples are functional, and the Ing have almost won. One Luminoth, U-Mos, stayed as a guardian of the energy when the others went into stasis pods. They knew that someone would soon come to banish the Ing.
The Space Pirates have discovered Aether's twin as well, and have been exploring it for mining operations. Dark Aether is rich in Phazon, a highly radioactive, mutagenic, and powerful energy source. Their mining operations have been hindered by Samus' twin, who continues to both steal Phazon and free Metroids from their energy-harnessing tanks. Soon, Samus herself must confront both her doppelganger and the Ing.
The gameplay is almost identical to the first. Controls remain unchanged, and you'll still be controlling Samus Aran as she finds power ups and solves puzzles. However, the poisonous atmosphere of Dark Aether tends to inhibit your exploration abilities. Because of this, when you are in this world, you will often be jumping from light beacon to light beacon to avoid damage until you get the more powerful suits.
The game is also a lot harder. Some bosses require you to fight in unprotected Dark Aether areas, making your dwindling health almost like a time limit. At least one boss fight is spaced a little too far from a save point to add artificial difficulty.
The game uses temples, similar to the Zelda series. Each temple has 3 keys, which must be gathered to face the boss, which then gives you an ability that lets you proceed to the next temple. This adds a strong element of linearity compared to the previous Metroid Prime game.