Upon first meeting the Tenth Doctor, he was awe-struck and saluted him, but as the operation went on he became more irritated by the Doctor's cavalier approach and dismissal of the military's tactics. When the Sontarans revealed themselves, he clashed with the Doctor when UNIT tried to launch a nuclear strike on the Sontaran warship, (TV: The Sontaran Stratagem) and was later forced by the Doctor to call a full retreat from the ATMOS factory when the Sontarans began slaughtering the soldiers present.
Mace coordinated the UNIT counter offensive to regain the ATMOS factory, having the soldiers rearm themselves with rad-steel coated bullets (preventing the Sontaran's field that negated copper bullets) and calling in the Valiant to clear the gas around the area. When beginning this operation, he finally snapped at the Doctor over his "lack of faith".
He personally killed Sontaran Commander Skorr with a pistol; rather than shoot the Sontaran from behind, he demanded that he face him before shooting him, showing that he was an honourable combatant. After the defeat of the Sontaran plan, he shared a kiss with one of his captains, Marion Price. (TV: The Poison Sky)