Picking up where Season 3 left off, we find McGarrett and Wo Fat cornered by unknown assailants in a detainment facility in season 4 of Hawaii Five-0, forcing McGarrett to break the law and turn on his own in order to find a kidnapped Catherine. Meanwhile Kono and Adam's secret location in Hong Kong is compromised, and Kono and Adam fight to stay alive when their hidden location is discovered.
After McGarrett and Wo Fat narrowly escape from maximum security, gunmen storm Five-0 headquarters, compromising Kono and Adam's secret location in Hong Kong and forcing McGarrett to break the law and turn on his own in order to find a kidnapped Catherine.
As the Five-0 team members investigate a murder, they are led to a Texas ranger who came to Hawaii to look for his daughter; Adam and Kono fight to stay alive.
Five-0 investigates the murder of a wedding crasher, and their prime suspect is the runaway bride. Meanwhile, Danny makes a big decision regarding his relationship with Gabby.
While working the murder case of an undercover ATF agent, McGarrett and the Five-0 team are tasked with watching Mary’s newly adopted baby after she gets sick.
McGarrett and Grover must work together to protect national security after they are tasked by the Governor to serve a warrant to a young computer hacker.
As Thanksgiving approaches, Five-0 investigates the murder of a secret service agent just as the President travels to Oahu. Also, McGarrett's Aunt Deb visits, bringing with her a shocking secret.
The attempted murder of a Pearl Harbor veteran leads Five-0 to use decades old evidence to investigate a deadly crime committed within the internment camps on Oahu during World War II.
Captain Grover is aided by McGarrett in his search for a friend who has disappeared, after it seems like he's committed murder. Kono persists in her quest to find Adam.
A real estate agent is found dead inside the walls of one of his listings. Meanwhile, Danny's mother makes an unexpected visit and brings with her some shocking news.
The investigation of a murdered pool cleaner leads the Five-0 team to a possible terrorist cell of radicalized students plotting an attack on U.S. soil.
When Grover's daughter is kidnapped by an old nemesis, he is forced to help steal $100 million in order to get her back. Meanwhile, Wo Fat escapes prison and heads back to Oahu.