The Dragon Riders assemble to relocate a Scauldron from the Vikings' fishing line. As Hiccup gives out orders, Fishlegs is quite worried about the actual mission. He states that the heat from the Scauldron's water blast could melt the scales off a Screaming Death, and he tries to use that fact to get the Riders back to safety. However none of them listen to him.
When the Riders are close to sneaking up on the Scauldron to capture it with their net, Fishlegs suddenly sneezes. The sneeze alerts the Scauldron of their presence and also sends Fishlegs spiraling out of control, bumping into Snotlout and throwing him off Hookfang. Snotlout falls into the net which forces the Riders to abort the mission.
Back at Berk, Fishlegs is still sneezing and itchy, he tries to tell everyone that he's fine but no one is convinced. So the twins do some tests on him to help get rid of his allergies, Hiccup then notices that Fishlegs is getting worse as he's standing next to Meatlug and realizes that he might be allergic to her. Astrid questions on how he can become allergic to her all of a sudden, the twins claim it to be "on-set allergies", which appear in your adult life. Hiccup, Snotlout, and Gobber take Fishlegs to Gothi hoping she can help, and she decides to hypnotize Fishlegs and tell him that he's not allergic to Meatlug. But before Gothi brings him out of it, Snotlout wonders if they can change Fishlegs into a fearless, arrogant, and bold viking, Hiccup tells him that they're not going to do that and tells Gothi to wake Fishlegs up. It turns out that Snotlout's words got through to him and Fishlegs becomes the viking Snotlout asked for; Thor Bonecrusher.
In the village, Thor Boncrusher walks around showing off to everyone, Astrid asks why Gothi can't change him back Hiccup explains that she can't do it without her staff (which Bonecrusher broke in half). As they're walking, the people become impressed by Bonecrusher's heroic deeds such as rescuing an old woman trapped in a burning house, saving a baby from a rolling cart of apples, and putting an out of control baby yak to sleep. The people cheer for more, to which Thor claims he will after he gets a weapon befitting of him.
At Gobber's place, Thor looks for a weapon of his own and takes the axe Gobber was working on, which also happens to be Stoick's axe. Hiccup tries to warn him that Stoick isn't the kind of viking who likes to share, but Thor doesn't care. At the mess, Thor tells the villagers about his skills.
- The idea of Fishlegs being allergic to Meatlug is possibly based on his book counterpart, who has an allergy to dragons.
- The egg that that Fishlegs eats look like a dirty, yellow Tide Glider egg. Specifically, the School of Dragons model.
- The twins appear to be well versed in allergies.
- Snotlout seemed to be 'infatuated' with Thor Bonecrusher, and the other riders find this hilarious, especially Astrid.
- Jay Baruchel as Hiccup Horrendous Haddock III
- America Ferrera as Astrid Hofferson
- Nolan North as Stoick the Vast
- Chris Edgerly as Gobber the Belch
- Christopher Mintz-Plasse as Fishlegs Ingerman
- Andree Vermeulen as Ruffnut Thorston
- T.J. Miller as Tuffnut Thorston
- Zack Pearlman as Snotlout Jorgenson