Captain Walker was killed when their ship crashed on the planet Krop Tor after encountering K37 Gem 5, the black hole the planet orbited. Zachary took command. (TV: The Impossible Planet) He helped aerate maintenance tunnels so that Danny Bartock could release a flare that disrupted the possessed Ood's telepathic field. He evacuated the survivors in a rocket, which the Tenth Doctor towed to safety with the TARDIS when the gravity field keeping Krop Tor in orbit vanished. He bestowed posthumous honours on his fallen crew, including the possessed Toby Zed, who Rose Tyler forced out of the rocket, and the Ood that were left behind on the planet. He was then able to return to Earth. (TV: The Satan Pit)