This was the most heartbreaking episode of the drama, because Hae Sung's brother died. His brother went after the leader who was then underwater fixing/drilling the pipe.
It was a suicidal task for he knew that he won't be able to survive the current once the drainage opened. There was not enough gas in the leader's tank and he gave his to him. It was a heartbreaking moment.
Before the accident happened, Ddol Mi was not speaking to Hae Sung. She was so hurt and mad when he refused to operate her father. As a daughter, she was only worried that he might die and being a doctor herself she felt frustrated that she was not confident to do the operation because she is not a surgeon who knows about those kind of operation. But it was a relief that she and Dae Gil was able to finish it when Dr. Yoo abandoned the operating table because he was so scared when things seemed to be critical and it scared him.