Nine-year-old Daniel walks into a local church with his parents, Marshall and Belinda. A seminary student escorts them to Monsignor Paul, a respected priest and believer in exorcisms. Daniel is laid out and restrained on an ornate credenza. When Daniel starts struggling, his entire demeanor changes, his face distorting into an unknown Wesen as he assaults the monsignor and seminary student and tosses the two grown men like dolls The end damage is a dead monsignor and a battered seminary student. Daniel is missing, much to his hysterical parents' dismay, but Nick quickly locates him crouching in the credenza's hidden compartment.
Dr. Sanderson's report reveals Daniel's immune system is abnormally stressed, resulting in an elevated white blood count. According to his parents, one year ago, Daniel simply transformed overnight into a violent child. No doctor or treatment has worked thus far - the exorcism was a last resort. Sometimes, Daniel's physical appearance alters, but neither parent is Wesen...
When the seminary student awakens, he insists Daniel is possessed by a demon. Meanwhile, a nurse sees "the demon" while attempting to take Daniel's blood. Nick orders a police guard on the boy's hospital door, not even allowing doctors and nurses to enter the room. Perhaps whatever is inside of Daniel only reacts to potentially violent situations...
Rosalee and Monroe suggest Daniel is a Grausen, a little-understood mutation or Wesen spirit that enters children; if allowed to grow to maturity, "possessed" children can wreak havoc on whole continents. As Wesen, Rosalee and Monroe are obligated to inform the Wesen Council of the dangerous Grausen. If they don't tell the Council, it's a death sentence for them. Royals and Grimms forged an agreement to allow Grausen children to "disappear," so Council member De Groot hands the dirty deed over to well-dressed operative Alexander - he knows what must be done to the child.
Meanwhile, Captain Renard and his bodyguard, Meisner, hide in a Vienna safe house while waiting for a Royal family meeting. Assassins shoot up the dingy basement, but Renard gets the jump on his attackers and kills them before escaping down into Vienna's sewage system. Elsewhere in Vienna, Adalind receives a mysterious phone call notifying her a car will pick her up tonight to go to "the house." The house ends up being the prince's accommodations - what does he want with Adalind?
Pouring over Aunt Marie's book, Juliette and Nick discover his Grimm ancestor believed Grausen were the result of a disease, not a possession. Daniel's parents remember he got the flu on vacation in Jordan one year and one month ago. Juliette hypothesizes they must isolate and kill off the protozoa - but how to kill an unknown protozoan? A frantic Monroe calls Nick to inform him the Council operative is on his way to the hospital now!
Nick, Hank, and Juliette show up to the hospital with their new theory but learn Daniel was released into his parents' custody 30 minutes ago. They hear a crash from Daniel's room. The operative dives out the open window after Daniel with Nick in hot pursuit. Nick tackles Alexander and leaves him in Hank's capable hands before chasing down Daniel. The boy's parents lead Nick to a homemade fort where Daniel lies shivering. Juliette realizes his hypothermic state might kill the parasite - a few scary minutes later, green goo rushes out of Daniel's nose and ears.
Alexander listens to Nick's account of Daniel's miraculous recovery, then takes the report back to De Groot. Concerned, De Groot comments that this is an entirely new species of Grimm they're dealing with...
Nick suspects a Wesen connection when a priest is murdered during an exorcism ceremony.