StoryRosenburg was hunted down by Edgar the Bug. He was able to stay hidden, but during lunch with a fellow Arquilian, he was found by the Bug. He then tells the Bug "you can kill us both but you will not find the Galaxy." At this point, he was killed along with his fellow Arquillian and the Bug stole what was thought to be the Galaxy. At the morgue, the two Arquilians and Rosenburg's cat, Orion are looked at by Jay and Kay with assumed identities and the morgue attendant Laurel.
Here, Jay accidentally opens the head, revealing the tiny Arquilian. He tells that the Galaxy is on Orion's Belt, and dies. After a long search, they realize that he meant that his cat's collar was Orion's belt. Moments later back at MiB Headquarters, Jay, Kay, and Zed find out that there is an Arquilian battle cruiser entering Earth's orbit. A message is delivered to MiB saying, "MiB deliver the galaxy or Earth will be destroyed! Sorry." Eventually Jay and Kay recover the galaxy after their fight with Edgar the Bug. Therefore the Earth is saved from total destruction.
Not much is known about Rosenberg save for his royal title and mission, but judging by his scenes prior to his death, he is shown to be a brave, kind, trusting individual. He loves his cat Orion and treats him like royalty even talking to him like a confidant. He's also kind enough to tip his cab driver despite being condescending to them for overcharging him. He also seems to be well loved and respected among fellow Arquilians since he greets a fellow member of his race with a hug and even offers a gift of diamonds for his children. He shows true bravery the second Edgar gets the drop on him by defiantly saying "You can kill us both but you will not find the Galaxy." He also shows determination, as he stays alive long enough in his damaged exosuit in order to warn the MiB. Despite only meeting J and Laurel once he confides to them, the only clue to saving Earth. He's also well respected among the MiB as his death shocks and saddens Kay who knew of Rosenberg and his title.
- Rosenburg was going to be a Baltian in first drafts of the film, and him and the other Arquilian would talk about ending their war. However, they made him an Arquilian too, and turned their coversation into gibberish which became Arquilian.