In 2368, a Borg sphere crashed on Planet 1865-Alpha, an uninhabited planet in the Delta Quadrant. The drones Two of Nine, Three of Nine (real name Marika Wilkarah), Four of Nine (real name P'Chan), and Seven of Nine, were the only survivors. Their link to the Borg Collective was temporarily severed, allowing them to regain their individualities. Lansor recalled working in mathematics before his life as a drone. P'Chan, Lansor, and Wilkarah intended to hide so that they would not be found when the Borg came to retrieve them. They went their separate ways.
However, Seven of Nine, terrified of her newfound aloneness, hunted each of them down and forcibly interfaced with each of them using her assimilation tubules. She caused the left parietal lobes of their brains to be transformed into interlink nodes that linked them together in an ad hoc collective, then erased the act from their memories. She then led them back to the crash site, where the Borg retrieved them.
Eventually, P'Chan, Lansor, and Wilkarah escaped the Collective, but could not break their ad hoc link, and thus were denied true individuality. In 2376, they sought out Seven of Nine on the USS Voyager in the hopes that she could help them, and discovered her responsibility for the link.
The Doctor, Voyager's holographic chief medical officer, found that the only way to break the link was to remove the interlink nodes. However, this would kill them within a month. Seven and The Doctor decided to perform the procedure, knowing that they would prefer a short life as individuals to being linked together.