During the examination of the Thing he is seen to make records of the whole process. We can probably conclude the fact he was the author of the most records which are later found by MacReady and Copper.
When Juliette-Thing tries to assimilate Kate and Karl is killed in process, he is together with Adam the witness of Juliette-Thing fatally mauling him and burning of the creature by Lars. During burning the remains of both dead bodies, Jonas seems to be sure that the Thing is still hiding among the rest of the remaining people.
He was later tested by Kate, a test that he passed because he had metal fillings. Later on he and Lars were tasked to bring in Carter and Derek in from the shed. When the two discovered that Carter and Derek were no longer inside of the shed, Jonas suggested that he and Lars should go back to Kate and the others, but Lars decided to look for the two prisoners. When Lars disappears, Jonas flees back to the others. When the others confront Carter and Derek, Peder prepares to burn the two, but is shot in the head, killing him instantly. When Edvard goes to check on Peder, Jonas notices that the flamethrower is about to explode; Edvard is knocked out from the explosion.
Carter orders Jonas and Colin to get Edvard to the rec room for treatment (We can notice that Jonas was willing to help Edvard before it was ordered to him contrary to Colin). They carry him with his arms slung over their shoulders, Jonas on the left and Colin on the right; when they reach the room, Edvard's left arm breaks off just above the elbow to form a Centipede-Thing and begins to assault a horrified Jonas. After the Edvard-Thing assimilates Adam and escapes the room, Kate sees Jonas in the process of being assimilated by the creature as it forces it's way into his mouth along with a large portion of his right hand merged with the Thing from his struggle with it. Jonas then gives Kate a pleading look: she puts him out of his misery and incinerates him.
- Jonas' death scene was similar to that of Windows' in that both men (after being hopelessly mauled by an imitation) were torched in mid-transformation; while leaning up against a bookshelf of board games in the rec room.
- Jonas assimilation via tentacles penetrating and merging with his face was also a reference to Garry's fate in the original Carpenter's film.
- Jonas confirmed that he always believed in the existence of aliens in his conversation with Adam. He shares this trait with Palmer from 1982 The Thing.