Prince Viktor pours a drink for himself and a wary Adalind. He's devastated by his cousin Eric's untimely murder - therefore, Viktor's retaining Adalind's legal services. In order to gain his trust, he charges her with composing a list of potential Portland murder suspects. What she doesn't know is that Viktor has installed hidden cameras in her hotel room, observing her while she rubs blood on her pregnant belly.
Back in Portland, a hulking man rummages through a nice house, stuffing expensive items into a rucksack. Suddenly, Scott, a young man fresh off a plane from college, opens the door. The house is a disaster zone and a quick minute of investigation reveals the massive intruder. Though the college student attempts to defend himself with a kitchen knife, he's no match for the robber, Gregorek, who promptly morphs into his reptilian Wesen form.
Nick and Hank's investigation is gruesome - the college student's arm was pulled out of the socket and blood spurted to the ceiling. Bloody footprints indicate the murderer's shoes are size 15 or larger. Some research back at the precinct tips off Nick that there might be a trend - this is the third robbery in that neighborhood. Harper's autopsy shows that Scott died from a broken neck, but his extensive injuries include multiple fractures and bite marks. The only similar injuries she's ever seen were from a shark attack...
Renard's new location is known only by him, Meisner and the castle spy. After he spends several days waiting in a safe house, the family calls on Renard. Upon their arrival at the family mansion, Renard and Meisner are led to the "trial" of the man who set them up and they watch as the torturer, Frenay, repeatedly asks for a woman's name. When the accused finally whispers it in Frenay's ear, he's promptly shot.
Meanwhile, a Portland storm drain maintenance team lowers a young worker, Jimmy, into a manhole. Alone in the storm drain, Jimmy calls out that he doesn't see the source of the blockage. Venturing further into the sewer system, he discovers a tree blocking the sewage, but Jimmy is thrown off by the sound of footsteps deeper in the tunnels. There's not another crew working on the blockage, so who could be down there? Up above, the rope goes taut while Jimmy's screams echo through the tunnels. His crewmates quickly pull Jimmy's limp, bloodied body out of the manhole.
Jimmy's five minutes in the storm drain proved fatal. The only similar injuries the paramedics have ever seen belonged to a bear attack. Wu is positive it's an alligator's bite marks. A freaked out Wu, along with Nick and Hank, follows the supervisor into the drains. Near the blockage, several personal items float in the water, along with Jimmy's missing leg.
At the precinct, the police officers identify the odd assortment of items pulled from the drains: all of the possessions belonged to people whose homes had been recently robbed, including Scott, the murder victim. The coroner's report reveals that the bites do resemble an alligator's teeth. There's only one glaring problem: alligators don't rob houses.
Of course, Monroe's heard the legends of alligator-type Wesen called Gelumcaedus. Nick and Hank bring Aunt Marie's book for a group story time session and Rosalee reads aloud that Gelumcaedus are one of the oldest known Wesen, dating back to the fall of Rome. Grimms use a special weapon against this Wesen called a vambrace, a metal forearm sleeve with a wicked-looking hidden blade. Nick is sure he's seen the instrument in the trailer.
Since Gelumcaedus have lived in underground tunnels since ancient times, it's a safe bet the rogue Wesen is hiding in Portland's 400 miles of storm drains. A city worker informs Nick that the only drain entrances are manholes and outfalls. Pouring over a drain map, Nick and Hank pinpoint outfalls near three looted neighborhoods. The Gelumcaedus must be using the outfalls as escape routes, which means he's probably hiding somewhere near the epicenter of the three entrances, near the spot where the storm drain inspector was murdered.
Armed with night vision goggles and Nick's vambrace, the two detectives search the tunnels for signs of life. Nick finds a door hidden behind moss revealing the Gelumcaedus' loot, loot that must have come from multiple cities. Perhaps he's killed people in other cities? Before they can investigate further, the hulking Gelumcaedus surprises Nick and Hank! He's no match for a Grimm, though, and he's dragged down to the police station for questioning.
The Gelumcaedus' name is Gregorek and he has a record: arrested in Salt Lake City and NYC for breaking and entering, but acquitted for lack of evidence. Monroe answers Nick's burning question: when he arrested the Gelumcaedus, Gregorek called him "decapitare," which is old-school Latin for "he who decapitates." At the station, Nick discovers that, based on the robberies' timetables, Gregorek had a partner. He calls Hank, who has already left the station for the night, to let him know that Gregorek had an accomplice, but the call comes a moment too late...
Hank is ambushed at his front door by another Gelumcaedus and dragged, unconscious, into the sewers. Nick receives a call from Gregorek's partner demanding Nick meet him at the outfall with his brother or find Hank in pieces. Nick follows the directions, leaving his gun in the car. When Nick sees his partner, Hank shouts a warning: there are three of them! With his handy-dandy vambrace, Nick pulls some serious Grimm moves and kills Gregorek's brothers, thus ridding Portland's sewers of the dangerous Wesen once and for all.
After a night of stealing Christmas gifts from cars, two young ruffians survey the plunder of their crimes. They're interrupted when a hulking beast in a Santa outfit emerges from the shadows. The visitor removes his hood to reveal large horns, a hideous face and a flaming forked tongue. It's the Krampus! Before the teenagers can escape, Krampus attacks them, easily catching one naughty kid and putting him in a large sack. The other thief manages to evade Krampus by hiding under a car. As Krampus departs with his hostage over his shoulder, he leaves his trademark calling card: a single lump of coal.
Meanwhile, in Vienna, Renard meets with an assassin and his cadre of rebels to discuss plans to take out his own family, The Royals, once and for all. Renard is the perfect weapon for the team of rebels: his half-Royal blood gives him access to key members of the family, and (most importantly) he has a Grimm at his disposal.
Back in Portland, Monroe and Juliette meet in secret at Monroe's house. Monroe informs Juliette that he has 42 boxes of Christmas decorations, and he wants to decorate the entire house while Rosalee is at work in order to surprise her with a winter wonderland when she returns home. It's a daunting task, but Juliette is happy to help.
Nick, Hank and Wu arrive at the scene of last night's incident. It's easy to see what the teenagers were up to. They find the second boy in a nearby car, alive but terrified. Later at the hospital, Nick and Hank interview the youth. The boy explains the events of the previous night: he and his friend were stealing presents when they were attacked by a horrific beast that kidnapped his friend. The boy can't explain who took his friend, but he freaks out when he sees a man dressed as Santa Claus. This is a shaping up to be a strange case indeed.
Later that night, another young petty thief picks the pocket of a woman out Christmas shopping and retreats to a nearby parking garage. Krampus attacks again, whipping the thief with a switch before bagging him up and disappearing into the night. A bystander witnesses the event and describes a nightmarish creature dressed as Santa. It's the worst time of year to be hunting a bad guy in a furry red suit. The detectives draw some unwanted attention by beating up a Santa in a busy shopping area, but they're no closer to finding the two missing boys.
Rosalee returns home from work to find that Monroe has decorated the entire house for the holidays. While it's certainly beautiful, it's also a bit overwhelming. Rosalee reluctantly confesses to Monroe that she's never really been very festive around the holidays. Christmas was a big deal when she was growing up, and the holiday always reminds her of her aunt and uncle, who died in a car accident on Christmas Eve. Monroe feels terrible, and Rosalee feels worse for ruining the Christmas spirit.
Rosalee wakes up the next morning to discover that Monroe has taken down all the Christmas decorations. She feels terrible about it. Nick and Hank arrive at Monroe's to get his take on the Santa kidnappings, and Monroe tells the detectives about Krampus. According to the legend, the Krampus is like Santa's evil twin. He kidnaps naughty children and hangs them from the tallest tree in the woods until he's ready to eat them on the night of the Winter Solstice. Coincidentally, that's today. The detectives don't have much time. If they can find the tallest tree in the area, they'll know where to find Krampus.
The detectives head to Council Crest, where they're hoping to find the tallest tree in the area. The gamble pays off when Hank finds a set of boot prints, and Nick's Grimm hearing leads them straight to the missing kids. Unfortunately, Krampus is right behind them. Nick intercepts the beast and a brutal fistfight ensues while Hank, Monroe and Bud race to free the kids. Fortunately, even the Krampus is no match for Nick's enhanced Grimm strength, and he knocks Krampus out cold with a single punch.
Nick can't take a hideous beast into the station, but luckily time is on their side. At the stroke of midnight, the Krampus transforms back into his human form. The man has no idea what happened, how he got into the woods or even how he got to Portland. He doesn't even know what a Wesen is, which might explain why Krampus has never been caught. Even Krampus doesn't know he's Krampus!
Nick and Hank know that they'll never convict the 42-year-old photographer of the Krampus' crimes, so they just have to let the man go and hope for the best next year. Monroe returns home to find that Rosalee has redecorated the house with Christmas decorations.
Alligator-like bite marks on a murder victim's body send Nick crawling into Portland's sewers in search of a violent Wesen. Nick and Hank battle the Krampus, a legendary anti-holiday Wesen who preys on the naughty youth of Portland.