Charlie is sick and needs Danny's help.
To complicate matters, Charlie has a disease called HLH and needs a bone marrow transplant from Danny. The disease is the only reason Rachel even decided to tell Danny the truth about his son. There's a big batch of mixed feelings brewing, but Danny showed a lot of maturity (we were surprised, too!) about the situation and didn't want Grace and Charlie to see their parents arguing all the time.
Watch Hawaii Five-0 Fridays at 9/8c.