Mack the Stray Dog is a stray canine encountered by protagonist Ted after Ted gets ejected from a movie theater. Mack reveals his ability to speak and exhibits a confident, adventurous nature. He quickly becomes Ted's companion, forming a trio with Ted and a discarded mechanical toy soldier called the Colonel. Mack spearheads their nocturnal visit to a closed amusement park, exploring its empty attractions alongside his friends. When security guards chase them, Mack aids the group's escape. They flee using fireworks rockets that launch them into a countryside forest.

In the forest, Mack accompanies Ted and the Colonel as they meet Professor Lemuel Gulliver. Mack joins the subsequent space voyage aboard Gulliver's rocket ship to the Planet of Blue Hope. There, he witnesses the conflict between the planet's original inhabitants and invading robots from the Purple Planet. Mack dives into the climactic confrontation against the robotic antagonists. After the successful liberation of the planet, Mack sees the inhabitants regain their human forms. The adventure concludes with Ted awakening back on Earth. Mack reappears, reverting to an ordinary, silent dog, accompanied by the now inanimate Colonel toy. This suggests the entire space journey may have been Ted's dream. The trio walks together down the street at the story's end, implying a continued bond despite the apparent return to reality.