The title is a play on words and the scientific event known as Brownian motion.
The story is concentred on the relationship between Hashida Suzu after 22 years living in this world, a young Tennouji Yuugo which his mission (given by the commitee of 300) is to investigate over her, and Tsuzuri Imamiya who married Tennouji during the story and have a daughter with him, Nae.
The events of the story begin the 4th July 2010, after some random talk with the gadget lab members, he visits the graveyard of Hashida Suzu with Nae and remember the person who gave him everything when he came to Japan.
We look back in time and see a younger Yuugo Tennouji the 22nd December 1996, his new mission is to go to Japan and investigate over a strange woman, Hashida Suzu. Her past before 1975 is unknown, she possibly hack SERN and she can have in possession one of the remainning IBN5100 in the world. Yuugo just met her by chance just after his arrival in Akihabara after being knocked off by her for being a molester over Tsuzuri Imamiya. After the confusion is dissipated she offers a cheap rentroom to help Yuugo to his installation in Japan. He also ask her to let him use the first floor of the building Suzu owns to open a business for repairing and selling obsolete items.