Messner lands on the human colony Fehl Prime in 2185, posing as a traveling hardware merchant. His feigned admiration for Lieutenant James Vega and the Alliance Marines secures initial trust. Secretly, he operates for the human-supremacist group Cerberus, investigating human colony disappearances tied to the Collectors.

Messner ferries Delta Squad in a M35 Mako to probe a jamming signal. After they destroy the alien device causing it, he proposes deploying the colony’s defense cannons against the Collector assault. The strategy collapses, driving them underground. There, Alliance records expose him as a Cerberus infiltrator stealing lab research. Messner concedes his mission studies the Collectors but claims alignment with Delta Squad’s survival.

During an assault on a Collector ship aboard krogan Brood’s vessel, Messner turns on the team. He executes Brood, triggering a crash that captures Delta Squad. Aboard the Collector ship, he confesses planting the alien device to lure the Collectors—sacrificing Fehl Prime to gather tech and motive intel for Cerberus. He compels asari scientist Treeya to interface with a Prothean archive, where she sees the Protheans' transformation into Collectors and their human-Reaper project. Messner copies this data onto a bracelet.

Deemed expendable by the Collectors, he bargains for his life by threatening to leak the data to the Alliance. Treeya snatches the bracelet mid-confrontation and flees in an escape pod. Cornered by Vega, Messner pleads for his life, offering intel for evacuation. Learning Treeya holds the data, Vega leaves him behind. Collector forces kill Messner.