Official story and features:
"When Samus investigates an abandoned Federation ship on a planet named Aether, her ship is incapacitated by a freak electrical aberration in the atmosphere. Crash-landing on the planet's surface, she finds the scene of a slaughter. A group of creatures borne of darkness, known as the Ing Horde, attack her and steal a number of her Power Suit's abilities, leaving her for dead. As she soon comes to realize, the planet has been dimensionally doubled: One exists in darkness, the other in light. The peaceful people of Light Aether, called the Luminoth, are slowly being destroyed by the Ing Horde, and the loss of their generators will mean the permanent eclipse of the planet by Dark Aether. Not only that, but Samus soon finds that something more dangerous than the Ing stalks the planet surface: A dark creature is on the loose, and growing more lethal by the minute.
Immerse yourself in Light and Dark Aether as you explore massive, dangerous alien environments teeming with intelligent enemies. Using an array of high-tech interfaces, analyze clues in the environment and even pinpoint secrets. Your Scan Visor will be integral, helping you explore every nook and cranny and scan creatures for vulnerabilities. Tons of new power-ups await, including the Dark and Light Beams, which make use of an all-new ammo system! Plenty of beam weapons, visors, bombs, missile and suit upgrades, and power-ups like the Space Jump and Grapple Beam return as well. Bounty hunter...or hunted? For the first time in the history of the celebrated Metroid franchise, up to four players can battle one another as they search for weapons, grapple across ceilings and turn into Morph Balls to make their escapes."
From Nintendo press release