Sometime in the period between 5 BBY and 4 BBY, the rebel group known as the Spectres embarked on a mission with the objective of stealing Imperial research containers. These containers held biological weapons and were located within the Kessel Bunker situated in the Kessel system.
The Spectres rebel cell began with the addition of the adolescent force-sensitive Ezra Bridger in 5 BBY. Following this, the team undertook numerous missions across the galaxy to challenge the Galactic Empire and disseminate an anti-Imperial message. Whispers circulated about the Empire's biological weapon experimentation at the Kessel Bunker, a laboratory located within the Kessel system. Having caught wind of these rumors, the Spectres journeyed to the lab, intending to pilfer a container to examine the Empire's activities. The container they ultimately chose was a climate-controlled one with freezing capabilities.
As the Ghost approached the Kessel Bunker, pilot Hera Syndulla skillfully maneuvered around the scattered debris. The Ghost sustained damage from the debris, forcing the astromech droid Chopper to perform rapid repairs on the vessel. When Hera transitioned to preparing for the climate-controlled crates in the ship's hangar, she delegated navigation duties to Chopper. After successfully navigating the remaining debris field, the crew arrived above the Kessel Bunker. He detached from the Ghost, secured a crate, and ascended back to the ship. However, the crate suffered damage and began to release mutant bacteria. Chopper used a welding torch to seal the crate, preventing the bacteria from harming any crew members. Shortly thereafter, Imperial forces detected the Ghost. While Hera repaired the Ghost's radar array, Chopper was once again instructed to pilot the Ghost. Subsequently, Chopper assisted fellow crew member Garazeb Orrelios in neutralizing the remaining TIE fighters. This engagement, however, resulted in a breach in the cargo hold, and Chopper was assigned the task of securing the freezing crate.