WARNING: If you haven't read the first seven chapters, I recommend you not to read the following explanation because that would be a spoiler for you. 

SO FAR, 

SOWON and SINB :

Sowon and SinB arrive in the Sun Kingdom after fleeing from the collapsing Mountain Kingdom. As a result, Sowon recovered completely from the dark magic that was killing her in her kingdom. SinB however was already "dying" because of the kiss she shared with the "dark" Sowon when they were in the Mountain Kingdom's Kingdom (see Calm Snow for further details). 

Since then, Sowon's new goal is that SinB wake up (Sowon is in love with SinB). She thinks that going to the Sun Kingdom will help SinB because she was told that in this kingdom the magic has no power. BUT still, SinB doesn't wake up. They arrive in Shamash Polis which is the capital where the queen lives. 

There, they meet Yerin the Sun Kingdom's queen. Sowon and Yerin already know each other because when she was younger Sowon and her family visited Yerin's family. 

SinB eventuelly wakes up thanks to Eunha (so far we still don't know how). BUT SinB is different because she suffers. Whenever Eunha is far from her, she forgets who she is and that leads her to panic. Plus, her eyes are slightly red (it's an important detail).

Yerin tries to help Sowon to heal SinB but it is difficult as they don't know what she really has. Yerin has given an unknown beverage to SinB which is presumed good (but again we didn't witness any good effect on SinB so far) 

UMJI and YUJU : 

Now in parallel, we have the story of Umji and Yuju. 
 

In Calm Snow, we learn that Umji is living with Yuju a mere hunter. Umji don't know who she really is until she meets her big sister Sowon. Before that, we know that she falls in love with SinB (even though the latter keeps leaving her side) and Yuju is in love with Umji. Again see Calm Snow for further details. 

Yuju and Umji both flee the Mountain kingdom because of the dark magic destroying the entire land. They arrive in the Sun Kingdom. Umji heart is broken when SinB decides to stay and die with Sowon in the Mountain Kingdom. Once in the Sun Kingdom, they arrive in a camp where they meet the Fremen (that's how Yerin calls them). We know little about the Fremen except that they have red eyes, are spiritual and that they hide themselves as they are persecuted by the queen. 

Umji and Yuju witness a scene during which a fremen behaves weirdly as he approaches Eunha (so far we don't know why) then he is  roughly killed by a guard. Umji and Yuju also meet SinB again however she don't reconize them. Because SinB don't reconize them, Umji thinks that SinB is drugged by someone in the castle. Umji buried feelings for SinB resurface and her new goal is to save SinB. So far, we don't know Yuju's goal toward SinB but what is sure is that Yuju will always prioritize Umji's