In December 2499, Bernelli found himself on Mar Sara as part of Second Squad of the 417th, dispatched to Oasis Station to retrieve a piece of Confederate equipment. During the recon he was ordered alongside Third Squad under Sergeant Jensen, paired with Private Ekart and PFC Ardo Melnikov. The so-called "recon" turned into a fight with the zerg after Bernelli found Melnikov who had fallen into a pit. He was ordered with Ekart and Private Xiang to frag the hole with explosives. Bernelli was later ordered to set a charge in order to divert other zerg attackers.
Bernelli managed to survive the entire phase of the battle though when he and the remnants of the platoon came back, they found their base of operations completely abandoned.
Lieutenant Breanne later ordered Bernelli on watch alongside Xiang, Cutter and Mellish to keep watch if any zerg came up. Given his position, he was absent during a mutalisk attack on the command center though soon after, he and Mellish saw a large number of zerg heading west of the base, where Mar Sara City was located.
Bernelli survived the initial battle in keeping the zerg occupied, though was later forced to retreat to the last remaining bunker. Bernelli and the others fought to the last, saving the lives of thousands of Mar Sarans.
Bernelli's personality steadily changes throughout his time on Mar Sara. At first, he was a generally confident individual, though given that he was firing at both real targets and "imaginary" ones too in Oasis Station, he had little ability to cope with stress (or simply a rotten shot). He further lost conviction during the time at the Bunker Complex and while his spirits perked temporarily in the final fight against the zerg, they were shot right back down. When last seen, fatalism seemed to have taken hold of him completely.