Hawaii Five-0's Danno is ready to make up for lost time with his son.

'Hawaii Five-0' Season 6: It's Time For 'Romance', Chin's New Love Interest Revealed

Season 5 brought about the shocking reveal that not only is Grace's (Teilor Grubbs) little brother actually Danny's (Scott Caan) son, but the youngster is deathly ill. Little Charles needs a bone marrow transplant, and his biological father could be the best match. Now that Rachel's (Claire van der Boom) closely guarded secret is out in the open, it's a safe to say Danny's life will never be the same.

"There's a nice little [storyline] of Danny having to take some time off to do that [bone marrow transplant]," executive producer Peter Lenkov told TV Line.

'Hawaii Five-0' Season 6: Cast Gets Down & Dirty In New PHOTOS From Set

Viewers will get to see father and son really hit it off during the series' annual Halloween episode. Charles may be stuck in the hospital, but his dad will make sure he still has plenty of fun.

"As grumpy as he is, with kids you see a whole different side of Danny," Lenkov teased.

Meanwhile, Grace has yet to learn of her brother's true parentage. How will the preteen feel about being lied to for the last four years?

Catch up with McGarrett (Alex O'Loughlin), Danno, and the Five-0 crew when Hawaii Five-0 returns to CBS on Sept. 21 at 9 p.m. ET.

Tags: Hawaii Five-O television CBS