There has been some discussion around the fourth biome in Returnal in relation to how it is essentially the only save point in the game. There are undoubtedly some questions here that I will aim to answer. There will be some spoilers at the end of this post which I will clearly add a warning for anyone who has not played the game or has not gotten past the third boss yet.
The fourth biome is unlocked after defeating the third boss of the game. When you get here you will find that any death from here on, will result in you respawning here instead of going back to the very first biome. This is essentially a checkpoint and the only save point in the game.
Since the obvious goal is to complete all of the biomes and finish the game, this checkpoint is a welcome addition for many who do not like the idea of grinding through the first biome forever.
Once you get to the fourth biome, it is essentially a new game as you do not go back to the previous biomes again.
Note: It is spoiler time. Stop reading now if you do not want to know the reason WHY the game allows you to respawn at the fourth biome instead of the first
The Story Explanation Behind The Fourth Biome
This is your second warning, spoilers ahead!
When you defeat the fourth boss you will get a fairly long cut scene. You are able to call for rescue as the enemy here that was blocking your signal is now dead, allowing your broadcast to make it off the planet.
Your rescue is successful and you are returned home, to Earth I am assuming. You see lots of scenes of a piano being played which shows your character getting older every time and at the very end, you see a gravestone. What this montage is doing is showing that you returned home and lived out the rest of your life before dying.
The catch here is that you died! This means that although you escaped the planet, you did not break the live, die repeat cycle. You escaped but were still tied to the loop. You returned home and died of natural causes from old age and that kicked the loop back in to return you to the planet as a young woman again. The challenge now is to try and figure out why you are stuck in this loop.
Why Is Helios And The Planet Different After The Third Boss?
When you respawn after the cutscene that ends with the fourth boss, you will return to Helios, your ship, that crash-landed at the start of the game. The ship has rusted and is covered in vines and alge. A lot of time has clearly passed, just like the character exclaims as you explore during the first respawn. But why, why has this happened to everything?
This concept is a little more tricky but the loop is not a time-travelling loop. It is more like a respawner that respects the time that has passed. Say you spawn at 1pm, die at 1:30pm, when you respawn, it will be 1:31pm but you will not have aged, just the world around you will have.
When you died of old age, you lived out the remainder of your years, which if I remember correctly, was over 60 years. The fourth biome is the exact same location as the third biome. The only difference is that 60 years have passed and this has altered the environment.
Is the Fouth Biome the Same Place As The First?
Yes, it is the exact same location. Helios is here and this is the location that you crashed in from the first biome. 60 years have passed since the first biome and the fourth which is why everything looks so much different.