Samus, now slightly Metroid herself, was able to regain her strength and successfully eradicated the X, including the fearsome SA-X, overcoming herself at her previous peak.

The Federation cannot rest until all Metroids have been exterminated, thus Samus is sent to their homeworld, SR388, to wipe them out for good. In her final battle on SR388, Samus witnessed the birth of a Metroid hatchling. As a result, the creature bonded with Samus and adopted her as its mother.

My clock currently says 4 hours, but I've actually played around 7. I find it interesting that the actual real life playtime isn't being talked about with this game. Seems like it's more like 20 hours from the few sources I've seen talk about it.