"I think he's going to surprise a lot of people," co-executive producer Paul Zbyszewski tells TVGuide.com. "He made a conscious effort to really say, 'Look, don't feel like you guys need to write comedy stuff for me.' He wants to expand his repertoire; he doesn't just want to be pigeonholed as wacky comic relief. So, while he does have some fun moments on the show, he's also got some dramatic stuff that he's got to carry in the episode."
Cook plays Matthew Williams, a hedge fund manager on Wall Street and the younger brother of Scott Caan's Danny. Although Matthew seemingly arrives in Hawaii for vacation and to spoil Danny's daughter, Grace, with gifts, Danny and McGarrett (Alex O'Loughlin) are informed that Matthew is the target of a federal investigation.
Naturally, that information proves to be quite a bombshell for Danny, who viewers will learn comes from a large, tight-knit family.
"We learn their family dynamic and how Matthew, being the little brother, has done really well for himself professionally and is sort of the pride and joy of the family," Zbyszewski says. "But there's no real overt hostility between these two guys. They have a great relationship, and while Danny's brother hasn't been out to visit him in Hawaii, these are two guys who are definitely not on the outs when Matt comes to the island."
Therein lies the rub: Despite his close relationship with his brother, Danny must keep what he knows secret or risk also being charged with obstructing the government's investigation. Since McGarrett and the rest of the team are busy tracking down a mad bomber who's picking off the children of judges and other political figures, Danny turns to his ex-wife, Rachel (Claire van der Boom), for advice — something he will likely have to do more than once this season.
As for whether viewers will ever see Cook's character again, Zbyszewski says, wait and see. "The way that we leave Danny's story with his brother remains open for perhaps revisiting it in Season 2, in terms of their relationship and how things work out," he says. "I don't want to give away the fun and the payoff of the episode, but I think people will be very excited to see what happens."
Hawaii Five-0 airs Mondays at 10/9c on CBS.