This is the story very fast written, a lot more is happening but i just put the most big part
S4: we left S3 with the story about ALIE that was saying the Earth was gonna die in six months bc of radiation. So the whole season is about finding a way to survive for 6 years on Earth without dying of hunger, radiation or water depravation. Along the way, the fight for commander is going nuts and we get a lot of people dying or almost dying: octavia, bellamy, nyliah,...In the end, Clarke saves Bellamy and 6 others person who end up in space, Octavia is the new "commander" (sorta without the black blood bc she won the conclave) and Clarke is left alone on Earth to survive by herself.
S5: After Apocalypse 2, three differents worlds exists: Clarke on Earth with a kid (another nightblood), Bellamy/Raven/Echo/Murphy/Emori/Harper/Monty in Space thanks to Clarke and Octavia and the rest of the clan and skaikru in a bunker. Bellamy and the rest dont have a way down so they're stuck in space, the tower collapse on the bunker so they're stuck and Clarke is alone on Earth. We get to do a time jump of 6 years in this season. An explorer from 100years ago that used cryosleep is back on Earth with criminal and we end up with another war. they all end up on Earth but with big mass casualties and crazy stories...
S6: Apocalypse 3 happened bc of one bag crazy guy so we ended the S5 with them leaving earth and Monty/Harper looking for a better life for everyone (who are in cryo), we left Monty/Harper having a kid and dying on the Eligius space ship. they woke up 125 years later close to a new planet in hope to do better and in the beginning it's a bit crazy with toxic stuff that make you wanna kill your friend and all...this planet is ruled by original explorer that are also IA and that are using the night blood as serogate and tell their world they're Gods! they try to do better but the first oppportunity the other guy has to use the body of the night blood Clarke, they do and put one of their God in but Clarke is not having it and fight back in her mind bc since she was a commander, a brain has a glinch... and they mess up the transition. we also get bellamy/clarke being friends again and closer than ever (for me they're more than friends but apparently from the creator, fans see things that is not shown) and we end up with another genocide and monty/harper kid being all weird and useless in the season
the end of this season was meh!
If i'd rank the season I'd say: S3, S1, S4, S6, S5, S2
I love S3/S1/S4 (probably my favourite of all)
the other 3 are not bad, just not as strong as the rest