“NCIS: Hawai‘i,” the CBS TV series inspired by the real-life Pearl Harbor-­based military crimefighting force, the Naval Criminal Investigative Service, enters into the second half of its second season operating like a well-oiled crime-fighting machine.

“It’s been super fun to have the first season under our belt,” said Vanessa Lachey, who plays Special Agent in Charge Jane Tennant, head of the fictional team. “That first season you’re always anxious wondering whether the fans will love it, if the studio and network is going to love it, and whether you’re going to be back.”

The show, which airs Monday nights, has charted some new ground in its short history. It’s the first in the popular “NCIS” franchise to have a woman as team leader, and its cast features diversity in a groundbreaking way, with two of the female characters involved romantically.

To open the new year, it’s pulling off another first. It will be part of a three-show “crossover” event, joining forces with “NCIS” and “NCIS: Los Angeles” in one three-hour storyline that will air at 7 p.m. Monday. “NCIS: Hawai‘i” opened its second season with a two-part crossover with “NCIS,” but this will be the first three-part crossover for the “NCIS” franchise.

According to CBS, the plot will focus on “a very personal, high-stakes case to take down a mysterious hitman.” The show begins when members from various NCIS divisions go to Washington, D.C., to celebrate their mentor’s retirement from the Federal Law Enforcement Training Centers (FLETC). He is found dead in an apparent suicide, but his former students are suspicious and launch an investigation, which takes the special agents to Hawaii and Los Angeles as well.

Christopher Silber, who along with Jan Nash created “NCIS: Hawai‘i,” said crossover shows are “always a lot of fun” but very challenging to create. The idea for this crossover came from CBS, and he and other show creators ran with it.

“It’s hard enough when you’re doing a two-way crossover with two shows,” said Silber, a co-executive producer and writer for several shows in the “NCIS” franchise. “You’re trying to coordinate logistics, actors’ schedules, with actors being available to their show but also available to your show so you can continue shooting.”

Long Zoom conversations and internal discussions were needed to plan the story, which Silber said “shows off the characters from all the shows, but is satisfying in its way.”

Hawaii itself will be prominently featured in the crossover, Silber said. “We’ll make it clear that we’re in a unique situation, and there’s no place like Hawaii,” he said. “Our agents oftentime lean on their knowledge of where they are to help them solve cases.”

Part of that reflects his own experience working on “NCIS: Hawai‘i.” “When I come over to work on the show, people are really generous about sharing (Hawaiian culture) with me and other people on the show, and we try to honor that as well,” he said.

For the actors, shooting a crossover with so many characters with a shared past involved some rather confusing research into everyone’s back histories, Lachey said. “That was the fun part for all of us, not only professionally on camera, but also personally,” she said. “We’re like, ‘Have our characters ever met? Did we ever go to FLETC together? And we would try to develop these storylines, so when we do the acting, it makes sense and is more believable.’”

What complicated matters further was that the episodes were shot in reverse order. “It was, ‘Wait a minute, how did we get here? Do we know how we got here?’” Lachey said. “And they were like, ‘No, that script hasn’t been written yet.’ ”

At about the same time this fall, Lachey was also busy with two Netflix projects, “The Ultimatum: Marry or Move On” and “Love Is Blind: The Reunion,” two romance reality shows that she hosts with her husband, singer and TV personality Nick Lachey.

“I’d wake up, and I’d tap next to me and say, ‘OK, Nick is here, so we’re either in Hawaii, or we’re on the road doing something for Netflix.’ And then my second question was, ‘What are we shooting today?’ ”

The second season of “NCIS: Hawai‘i” has had some other memorable moments for Vanessa Lachey. In the first season, Tennant’s ex-husband has a baby, and for the second season, the baby made an appearance in the show.

“I was like, ‘I get to hold a baby!’ That’s so special, because I have three children,” Lachey said. “And it’s crazy, because I’m holding the baby and I’m talking about North Korea. That’s kind of the beauty of Tennant, because she’s a mother and friend, but she’s also the special agent in charge here in Hawaii, and it’s all the hats that she has to wear.”

Lachey formed a deep commitment to the islands while shooting the pilot episode, when she had to jump into rough waters near the Halona Blowhole during a fight scene in the middle of the night.

“I was sitting there, soaking wet, with a blanket around me,” she said, “and I just looked at the ocean with my feet in the sand, and I looked up at the sky and started talking to the land. ‘Please take care of me. Please take care of me while I do this show, and I will do justice by you.’ ”

Shortly thereafter, she called her husband and had him bring their children to Hawaii to look for a home. Lachey, whose first starring TV role was in a pilot set in Hawaii for a series called “The Break” in 2003, saw her life coming full circle with the move here.

Beyond the crossover event, “NCIS: Hawai‘i” fans can expect a lot of thrills, chills and excitement for the rest of the season, which was nearly wrapped up as Lachey spoke in mid-December.

“We’re literally ending it with a bang,” she said. “We have had some of our most explosive and action-­packed episodes yet.”

—

WATCH IT

“NCIS” crossover event

“NCIS,” “NCIS: Hawai‘i” and “NCIS: Los Angeles” will present a three-hour storyline. Airs 7 p.m. Monday on CBS.