'Hawaii Five-O' season 6 episode 3 watch online: What is Catherine hiding from McGarrett?

Fans of McGarrett and Catherine's relationship will have something to look forward to in the third episode of season 6 of "Hawaii Five-O".

The episode is titled "Ua 'o'oloku ke anu i na mauna", which translates to "The Chilling Storm is on the Mountains", and it will see McGarrett gearing up to propose to Catherine. [Click here to watch the episode.]

However, it remains to be seen if the proposal has a happy ending, as Catherine has a secret that could potentially ruin her relationship with McGarrett.

According to a sneak peek of the upcoming episode obtained by TVLine, McGarrett catches her talking to someone shortly after their sexual romp, and the truth about something or someone she has been hiding in Afghanistan could spell trouble for the couple.

Here are a few theories fans have come up with regarding Catherine's mystery phone call.

"I bet she's going to end up going on another undercover op or something like that," one fan noted. And according to another, Catherine has been recruited by the CIA.

"Her ability to conduct an op by herself in Afghanistan, and her sudden unexplained reappearance in Hawaii, have something to do with Doris's involvement. And perhaps she isn't really just retired Navy anymore, but has been recruited by the CIA," the message reads.

Peter Lenkov, executive producer of the show, recently spoke to Entertainment Weekly about season 6, and according to him love in an important theme that will be explored in the current season.

"Every year we try to hit a theme," Lenkov said. "This year the theme is love — aloha — and we're going to get into everyone's love life a little bit."

"We ended last season with a wedding between Kono and Adam, and we're going to follow that honeymoon period," Lenkov said. "We're going to be playing people's personal lives first and foremost, in terms of character arcs. We're doing a Valentine's Day episode which is going to really hammer home that theme."

The official synopsis of season 6 episode 3 reads:

A scuba diver is killed with a gun that was used in a recent murder; McGarrett gets ready to propose to Catherine.