Answer:
The main plot event in Arsat's story is that Arsat and his brother run away with Diamelen.
Step-by-step explanation:
"The Lagoon" by Joseph Conrad tells the story of a man who tries to find the way of staying with his beloved one, in the excerpt the line ‘Go and take her; carry her into our boat.’ shows that Arsat's brother told him to take Diamelen the woman he loves so the tree of them could run away on their boat, then this is the main point event.