Magnus and Erin Hansen, Annika's parents, were exobiologists investigating the existence of the Borg. After a great deal of persuasion, the Federation granted the Hansens the use of the USS Raven, a small long range craft, to aid them in their investigation. In 2353, they took Annika, then aged six, along with them. They spent a good deal of time aboard the Raven in search of the Borg; Annika celebrated two birthdays aboard the ship. Eventually, the Hansens encountered a Borg cube and followed it through its transwarp conduit into the Delta Quadrant, the Borg's region of origin. They gathered a great deal of scientific data on the biology of Borg drones and the nature of the Collective, moving undetected through Borg space due to multi-adaptive shielding, invented by Magnus Hansen. They even went aboard Borg vessels, using bio-dampeners to remain undetected. However, their research came to an abrupt end in 2356 when an ion storm struck the Raven. The ship sustained damage, including, most importantly, damage to the multi-adaptive shielding, which went off-line for 13.2 seconds. This left them exposed long enough for the Borg to detect them and perceive them as a target for assimilation. The Hansens tried to evade pursuit by masking the Raven's warp trail, but the Borg found them. They and their daughter were promptly captured and assimilated. (VOY: "The Raven", "Dark Frontier")
[edit] Life as a Borg drone
Borg drone Seven of Nine, Tertiary Adjunct of Unimatrix 01
Borg drone Seven of Nine, Tertiary Adjunct of Unimatrix 01

Annika Hansen spent the next eighteen years of her life as a Borg drone, given the designation Seven of Nine. As a drone, she assisted in the assimilation of millions, from individuals to entire species. She personally assimilated many individuals, from a variety of species, including Humans, Klingons, Ferengi, Bajorans, Bolians, Krenim and Cardassians. (VOY: "Infinite Regress")

In early 2368, Seven of Nine, along with three other drones in her unimatrix, crash-landed on a planet in the Delta Quadrant. The other drones, who were assimilated as adults, began to regain their identities upon being severed from the Borg Collective, but Seven was frightened as she knew nothing else but life as a drone. She forcibly linked the other drones together into a temporary collective in order to suppress their identities, and they were retrieved soon after. (VOY: "Survival Instinct") Seven of Nine remained a Borg drone until 2374, when she was liberated.