Hawaii Five-O” fans may be singing Christmas carols at home, but for the fiercest crime-fighting force on the island, there's not even a hint of tinsel to be found. In this week's December 13 Season 10 fall finale, "Ka i ka 'ino, no ka 'ino" (To Return Evil for Evil), what seemed to be a shadowy turn by Adam (Ian Anthony Dale) into the tactics of the underworld (for the sake of getting his lady love, Tamiko, back) becomes a truly deep and dark plunge into Yakuza.

Hawaii Five-O” boss, Lt. Commander, Steve McGarrett (Alex O'Loughlin), is as mystified as everyone by the abrupt behavior of his team member.

Adam turned from his syndicated crime upbringing for the sake of love with Kono. Now, the inscription from his ex-wife on his watch puts him at the scene of a crime. He surrenders his badge and his weapon to Steve, but when it comes to the truth, he relates only that “I can't give you that.”

Danny (Scott Caan) is back on duty in this final episode before 2020, and he’s still staying with Steve. This time, it's not for his bereaved friend, but instead, because of a sewer backup, but he’ll still be “taking up all my clean towels” as Steve reminds. Danny chalks up Adam’s behavior to being in love with Tamiko, the daughter of the boss, Masuda, but Adam’s true motives may run much deeper.

This holiday season episode replaces Christmas cheer with a heavy dose of the creepy factor, and that includes an unexpected connection to McGarrett’s greatest nemesis, Wo Fat.

There's nothing holly-jolly to this one, but the suspense will give “Hawaii Five-O” fans a jolt.

Helicopter down and heavy-duty artillery

Danny was the one who brought Adam out of the drunken doldrums amidst his split from Kono and gave him a “Hawaii Five-O” badge. He asks Steve to give him a chance to talk to him before letting police justice fall.

Of course, Adam isn’t answering his calls. Adam is standing firm in consoling Tamiko and also standing firm in the knowledge that Kenji (Fernando Chien) was the killer of Masuda, her father. When Kenji tells Tamiko that she can take comfort in knowing that the killer didn't survive, Adam tracks him outside, and assures him that the “performance” was not the truth and that he has no more leverage because now “Noshimuri” is not part of “Hawaii Five-O.” Kenji retorts that Adam is now a weaker target because of everyone he cares about, gazing at Tamiko.

Adam definitely has a dangerous plot in mind. He is doling out big rolls of cash for cars with “no paperwork” and aged Japanese whiskey.

The other members of the team are taking on a case of a tourist helicopter being shot down, and not by an average weapon. A shoulder-fired missile took down the aircraft, and there were a lot more where that one came from. The weaponry is traced to the same Russian make of arms that Wo Fat was known for, and tracking sources after so many years since the villain’s demise would be difficult. Who they could track was the helicopter pilot, who inexplicably canceled a full day of tours before the takedown.

On record, his reasons were for maintenance, but in reality, he was on a special mission near the mansion of another very wealthy connection to Wo Fat, named John Tang.

Tang inherited the motherload of Wo Fat’s weapons cache, and they were well stored at the residence.

Danny and Steve go to survey the site from the air, and see bodies everywhere. The retrieval operation was so well-planned that guards did not even have a chance to draw their own weapons. Junior (Beulah Koale) had a elusive encounter with a strange woman at a gas station. She admired his 1985 El Camino and gave lingering looks to him as she left. He noticed that a man in the surveillance video of the heist was the same man who drove her SUV. There were no leads on the man, but plenty of “red-sealed” records on the woman, who went by several identities, but was known as Dayo Mei (Eugenia Yuan).

The character makes her first appearance as an unassuming elderly lady, toddling to her condo room. She quickly goes into the bathroom and pulls off prosthetic makeup to reveal herself, before burning the evidence.

In the interim, Adam hears Tamiko ponder whether vengeance isn't the “simple” approach to death and grieving. He asks how well she knew her father's men, no doubt leading to the revelation about Kenji. In another scene, the couple is toasting with the expensive whiskey, and she asks, “Are you sure you want to do this?” before Adam is called behind closed doors for what looks to be a Yakuza induction.

Unholy matrimony

Dayo Mei has a long history of escaping from women's prisons, but her identity is still truly unknown to “Hawaii Five-O.” Along the way to finding this villainess, some interesting characters appear, such as a “money mistress” to one of the helicopter passengers.

She declares that some rich men like to relinquish control of their money before burning one of Danny's $20 bills. He did not enjoy the process.

The team traces the address of a shell company and tries to intercept the criminals, engaging in one of the drama’s famous car chases through the middle of Oahu. Little do they know that Mei has just completed a huge deal, and Tani (Meaghan Rath) remarks that “I'm not seeing any weapons here.” The surveillance tells Steve they are in the right place, and a massive shootout ensues, with Mei displaying her delight in mass casualties. She taunts one of the victims, saying that she'll be glad to call an ambulance for him, averting death by just minutes, if she will give rescuers a message.

She asks that he tell them that Wo Fat’s widow can't wait to meet Steve McGarrett, in a tone as icy as winter’s chill, just before credits roll. Eugenia Yuan is fit to be as delectable and ruthless as a foe as the actor who played her character’s hubby, Mark Dacascos.

Some fans will get their dream wish on January 3. That will be the first part of a “Hawaii Five-O”/ “Magnum PI” two-part crossover event. Happy New Year!