The 100 season 3 episode 7
Tension between the Grounders and Sky Crew rises in The 100 season 3 episode 7 The CW

After Charles Pike's army attacks the Grounder's village, the 12 Clans go to war with the Sky Crew. The only person standing between them is Lexa, who calls out against attacking Arkadia in the 100 season 3's upcoming episode.

Episode 7 is titled, Thirteen, which will air this Thursday, 3 March at 9pm EST on The CW. The episode will also have a flashback scene of the nuclear apocalypse, and how it happened. Click here to watch the episode online on The CW website (only in the US).

Executive producer Jason Rothenberg previewed the upcoming episode in a promo. According to Rothenberg, the episode will feature an origin story. "It's a flashback to before the war, in fact, we even see the apocalypse, that created the world as we know it as the 100, happened from the vantage point of people that were on space stations at the time."

Meanwhile, on the ground in Polis, Lexa and Clarke have become close said the producer, and added, "We have this two great stories, we have this Grounder political continuing conflict between the Sky Crew and Polis and the 12 Clan and how they all fit together, and we have this AI [Artificial intelligence] story."

"This is the episode where the two shall meet, and realise at last what it all means in a mindblowing way," teased the showrunner.

The synopsis for the episode reads as follows:

Lexa (guest star Alycia Debnam-Carey) tries to maintain peace and order within the clans, and Clarke (Eliza Taylor) uncovers a strange and game-changing truth. Meanwhile, a flashback takes us into the darker chapter of humanity's past.

Do not miss the mind-blowing episode of The 100 season 3 tonight on The CW.