Metroid: Lambda Squad is a game that takes place ten years after the events of Metroid Fusion. This game doesn't revolve around Samus Aran, but rather a new generation of heroes destined to replace Samus as protectors of the galaxy, called Lambda Squad. The story revolves around six youths, including Richard the leader and rifleman, Joseph the heavy-artillery man, Jenny the sniper, Mariana the scout, Billy the demolition-expert, and Alexis the medic, as they are sent to the Planet Doleous to stop Samus Aran and her army of space pirates from unleashing Metroid Supreme. It is the first in the Metroid series to not utilize Samus Aran as the protagonist.
Metroid: Lambda Squad utilizes a squad-based tactical system that allows players, controlling the leader Richard, to give orders to their five squad-mates to perform certain tasks, like assault, hold-back, defend, or follow. Each member of the squad has a certain specialty: Richard uses an assault rifle and can give orders to his squad; Joseph can attack larger enemies with his giant machine-gun, which can transform into an RPG-launcher; Jenny, with her sniper-rifle, can attack enemies from long-range; Mariana, the scout, can utilize a pair of submachine-guns and can turn invisible for a short period of time; Billy can wield a shotgun and can toss sticky-bombs on any surface; and Alexis utilizes her small pistol and can utilize medkits and stimpacks to heal and buff her allies.
The game still carries some of the trademark exploration, enemies, and characters such as Samus, the [chozo], [Ridley], [Kraid], and so on, which the series have always been known for. Because the player is with a squad, there is none of the trademark sense of isolation within the game, as players have to utilize squad-based tactics and teamwork to complete their goals. It also utilizes six-player online co-op, allowing each player to play as one of the members of Lambda Squad, as players have to work together to solve puzzles, explore different rooms, hallways, and corridors, and fight off enemies, provided that they trade friend codes with each other.
Story (spoilers ahead)
Right between the events of Metroid Fusion and Metroid: Lambda Squad, Mother Brain, assimilating herself in a new brain-like console like the one in Metroid and Super Metroid, abducts and genetically alters six young children into her super-soldiers, built to help her eliminate Samus and conquer the galaxy. However, Samus frees the six children and defeats Mother Brain at the cost of her life. Ever since then, the Federation, learning of their potential, adopts the six children as their own and makes them into their new super-soldiers, built to replace Samus as protectors of the galaxy.
Ten years later, Samus Aran returns from the dead as a space pirate as she captures a small metroid cell sample from the Federation lab and plotted on doing something with it. The Federation sends the six children, Lambda Squad, who are now teenagers, to find Samus on Planet Doleous and stop Samus from unleashing whatever she was planning with the stolen metroid cell sample. As they travelled the underground base within Planet Doleous, Lambda Squad had their scout, Mariana, crawl through an air-duct to spy on Samus. What they discovered through what Mariana saw was that Samus was developing a giant, mutant metroid. However, Samus spottted Mariana in the air-duct, and fired her cannon at her, forcing the rest of the Lambda Squad to head over to the room Mariana was in to rescue her. As they arrived to the giant metroid's room, they discovered Samus standing above an unconscious Mariana, as she described to them why she was doing this. Samus formed her own space pirate army to free the metroids of the galaxy from the Federation, by combining the metroid cell she stole with the energy, Phazon, to create Metroid Supreme. Once she unleashed Metroid Supreme on the galaxy, Samus would be unstoppable. Just when Lambda Squad and Samus' space pirates were about to fire their guns at each other, Mariana woke up and described to her team that Samus is both herself and not herself, though she has no idea if she really is Samus. Samus grabbed Mariana and used her as hostage to protect herself from Marian's teammates. However, that didn't stop Mariana from activating her cloak, turning herself invisible to hold Samus down long enough for the others to escape and warn the Federation about the threat. As the five squad-mates ran away without Mariana by their side, by the time they made it to the elevator, Billy chose to stay behind and use one of his sticky-bombs to blow up several of the space pirates alongside himself while he gets his teammates out. The four remaining squad-mates return to the cavern entrance, but were surrounded by more space pirates led by none other than a newly-resurrected Ridley, who returned to life thanks to Samus' biotechnology. Joseph then chose to leave himself behind to fight Ridley, distracting him long enough for Richard, Alexis, and Jenny to escape. As the three remaining members of Lambda Squad escaped from the cavern, they hid under a pile of rocks to evade capture from the space pirates. They were left without Mariana, Billy, and Joseph, as well as any hope needed to defeat Samus.
Richard, Jenny, and Alexis calls their Commander Sonia on their communicators about the current situation and requested the aid of a fleet. However, Sonia refused, and wished that they'd defeat Samus and Metroid Supreme by themselves. Alexis grew frustrated not only because they lost their comrades, but also because she regretted joining the Federation to become their killing machine. Richard then comforts Alexis.