After a Marine's wife is kidnapped for ransom, the NCIS team uncovers dark secrets as they race to track down her abductor before it's too late. Meanwhile, Sam seeks Ernie's expertise decrypting a computer program that may have deadly ramifications.

When a high-value bioweapons expert is murdered in a secure facility on Hawai'i, NCIS and the NCIS Elite team join forces to track down the killer and discover an even deadlier threat.

Following a devastating ambush with a deadly bioweapon, the NCIS team must track down a mysterious terrorist group before they can strike again.

The NCIS team investigates the death of a Navy sailor and goes head-to-head with a ruthless local criminal organization. Also, Lucy looks for a new apartment.

When a Marine corporal dies in a tide pool after being exposed to fentanyl, the NCIS team must quickly find the source of the drugs; Alex talks to his parents about taking a gap year; Ernie reveals personal news to Lucy.

When Commander Chase is abducted from her home by an Army Ranger accused of murder, the NCIS team must act quickly to find her and the suspect.

An experimental Naval aircraft crashes on Oahu, and Tennant and her team must find who is behind it before classified state secrets are exposed.

Tennant and NCIS must take down a notorious team of international thieves using WWII explosives on armored trucks before they complete their final big job and disappear. Also, Kai searches for a new place to live; Lucy and Whistler have a standoff about their previous encounter; and Ernie's mission in the field is more alarming than he expected.

Kai goes undercover with one of Hawai'i’s oldest surf gangs when a petty officer who’s trying to help wayward kids find a new path in the Marines is murdered.

When a marine is shot, the team investigates. They start by talking to him, he tells them that he went riding and he got shot probably by a hunter. They check out his story and learn he wasn't telling the truth. They go back to see him but he left the hospital, not fully healed. They learn a relative came and helped him. They see the relative, who doesn't know much but they find a bag filled with drugs. They think the bag belongs to his son. So they try to talk to him but he disappeared. They think the boy was asked by a drug ring to deliver the drugs but he changed his mind which means his father is trying to protect him. But the gang will stop at nothing to get them, so they try to find them. When Kai's father doesn't see his doctor, Kai needs to put aside his feelings to help his father.

When a Japanese sailor is killed on American soil and evidence links the case to the previous murder of the victim’s girlfriend back in Japan, NCIS must find the murderer before the wrong person is accused and the case triggers a diplomatic crisis.

When a Navy petty officer is murdered, Jane and the team investigate while protecting the victim’s friend and colleague.

When an anti-capitalist protestor is murdered, Jane and the team investigate and find themselves caught in a war between eco-activists and a tech billionaire fighting over a piece of land. Also, Jane and Captain Joe go on a date.

When Tennant and NCIS are tasked with finding a kidnapped globe-trotting social media star, they find that she is not who her husband, or her followers, think she is. Also, the team is at odds with Whistler, who has an agenda of her own.

NCIS crosses paths with Whistler's team while investigating a shipping container filled with contraband weapons; Tennant debates arresting Alex's friend's dad, knowing it will cause her friend to move away.

While Jane is questioned following Maggie Shaw’s arrest, the rest of the team investigates a mysterious case of Marines with damaged ear canals from a weapon that emits ultrasonic waves. Also, Ernie visits his friend, Dr. Tony Lee (Alec Mapa), to identify possible suspects who had access to his classified weapon.

Kai goes undercover as a chef in a local restaurant to gather intel on a notorious criminal kingpin who has a connection to the eatery. Also, Jane discovers that a school on the mainland has recruited Alex on a baseball scholarship, which he's kept secret from his family.

When a ransomware attempt causes a dam to malfunction, Ernie and a team of hackers are tasked to find the culprit quickly, before all power and water is cut off on the island. Also, Lucy and Whistler work together, giving Whistler a chance to apologize to Lucy and mend their relationship.

The NCIS team investigates a shipwreck carrying exotic animals that now threaten the native wildlife on Oahu. Also, Alex sustains a career-ending injury and Kai asks Melanie, a fish and wildlife agent, out on a date.

Captain Milius returns to Hawai'i for a secret prisoner exchange operation and brings in Jane to be his personal security escort in the Philippines, where the exchange will take place.

When evidence to put away a drug kingpin is stolen, Lucy goes undercover at an underground poker tournament to find out which high-roller is behind the crime. Also, Lucy’s feelings are in turmoil when Whistler’s ex-girlfriend arrives in town.

When NCIS investigates the mysterious death of a Navy engineer, Joseph Chan, they learn the last person he met with was his colleague, Maggie Shaw, Jane’s mentor and friend, who’s been kidnapped. Also, David Sola, a New Zealand intelligence service case officer arrives in Hawai’i, following a lead that connects Joseph’s death to a Chinese black op secret agent.

While Jane investigates Maggie’s kidnapping, she’s shocked when she learns the truth and enlists her team and Whistler to prove her findings.

NCIS Agents Nick Torres and Jessica Knight travel to Hawai'i when they learn a key witness in one of their old cases has turned up there with crucial evidence. The crossover starts on NCIS S19E17 Starting Over (I).

When a navy seaman is involved in a murder, the NCIS team is called to work the case on their day off. Also, Lucy finds out Whistler turned down a promotion in D.C. to stay in Hawai'i.