Kidnapping of Nebula

Background
"There's another conciousness sharing her network. Another Nebula."

- Ebony Maw to Thanos

Nebula and Rhodey were tasked with retrieving the Power Stone from Morag in 2014. Unbeknownst to the two, Nebula's cybernetic enhancements forced a link between her and her past self, due to a shared network. This allowed the Nebula from 2014 to access her future double's memories. Through this, Thanos from 2014 was able todiscover their plan. After Nebula retrieved the stone, Rhodey and she prepared to return to the present. However, as Thanos and Ebony Maw were still accessing her memories, Nebula was unable to return to the present. Through her link, she was able to see through her double's eyes, Thanos witnessing his own death.

Kidnapping
"Come in, we have a problem! Thanos knows! Thanos..."

- Nebula attempting to warn the Avengers, shortly before her capture

Nebula ran back to the ship and attempted to warn her teammates, but her ship was quickly beamed up by the Sanctuary II before she could contact anyone else.

Impersonation of Nebula
"You disgust me. But that doesn't mean that you're useless."

- Nebula to her future self

On the Sanctuary II, past Nebula beat up her future self. Nebula attempted to plead with past Gamora, warning her of her demise at the hands of Thanos. Past Nebula kicked her and took present Nebula's orange headpiece, something the past Nebula lacks. Past Nebula then went to Thanos and kneels before him, presenting the Pym Particles and now wearing present Nebula's orange accents.

Aftermath
Past Nebula returned to the present, the switch going unnoticed as present Nebula was already distant towards the rest of the Avengers. As Hulk used the second Infinity Gauntlet to snap the Decimated back to life, Past Nebula activates the time machine and bring the Sanctuary II to the present, causing the Battle of Earth. Gamora and Nebula later kill the past version of Nebula after the Sanctuary II destroys the New Avengers Facility.