Pondering this fact, I realize... I owe the Metroid hatchling my life twice over.
The Metroid hatchling, often referred to as the baby, is the last remaining Metroid from planet SR388. Samus witnesses the Metroid hatch discovered by Samus in Metroid II: Return of Samus, and it helps her escape the Metroids' home planet. Samus, rather than kill the Metroid in accordance with her mission directives, takes the specimen to the Galactic Research Station at Ceres, so that Federation scientists could study its energy-producing qualities. While she was leaving SR388, Samus was ambushed by Proteus Ridley who attacked her at the landing site in an attempt to steal the baby Metroid, but was eventually subdued. According to Samus, the scientists discovered that the Metroid could be used to benefit civilization, but exactly how is never known.
Shortly after Samus leaves Ceres, the scientists are murdered, and the Metroid is stolen by Ridley and taken to the reconstructed Space Pirate base on Zebes. After defeating Ridley in his lair at Norfair, Samus finds the Metroid's casing broken in two. She encounters the creature herself in New Tourian amidst dried husks of enemies. Since its time away from Ceres, the Metroid has grown to a gigantic size, far larger than any other juvenile Metroid Samus has encountered, but due to its existence away from SR388, it had not proceeded through the Metroids' growth cycle. The Metroid proceeds to attack Samus, draining almost all of her energy before ceasing and fleeing, possibly to warn Samus not to proceed any further.
A short time later, as Samus is overwhelmed by Mother Brain's Hyper Beam, the Metroid returns, attacking Mother Brain as the she prepares to kill Samus, and uses the energy taken from the monstrosity to heal Samus and empower her with the Hyper Beam. During this process, a rejuvenated Mother bombards the infant with beams fired from its mouth, killing it. Its remains rain down upon Samus, and she gains the Hyper Beam, using it to obliterate Mother and escape Zebes.
The Metroid hatchling was used to create the genetic basis for the failed Space Pirate experiment Mochtroids and their successful Metroid cloning later on. The Galactic Federation scientists managed to save some DNA samples from the hatchling as well. These samples were used to create unfreezable Metroids on the Bottle Ship, to create the X vaccine, Vaccine "Metroid", and to provide the basis for the secret Metroid breeding program at the B.S.L research station's Restricted Laboratory.
- The only flashbacks in the series that refer to previous game are those in the beginnings of Super Metroid and Metroid: Other M. In both of these flashbacks, the Metroid hatchling is the main focus.
- Though the hatchling only appears in the ends of Super Metroid and Metroid II: Return of Samus, its importance carries on through Other M and Metroid Fusion, meaning that its presence spans every game chronologically since Return of Samus.
- Though Samus may be heralded as the savior of planets or even galaxies and the universe, she gives credit to the hatchling for saving her life twice, initially against Mother Brain and again as the Metroid vaccine in Fusion.
- "I had gone to planet SR388 to destroy the Metroids. After a deadly struggle, I managed to defeat the Queen Metroid, and as I was preparing to return, a baby Metroid hatched before my eyes. As I was the first thing the baby saw, it took me to be its mother." Metroid: Other M Instruction Booklet, Pg 6
- "They proposed using a Metroid cell to make an anti-X vaccine. It seems that the Federation had managed to preserve a cell culture from the last infant Metroid from SR388. The serum was prepared and injected without delay. ... Pondering this fact, I realize... I owe the Metroid hatchling my life twice over." —Samus Aran (Metroid Fusion)