Ace Hart is the star of the animated segments on Dog City. Referred to as "Ace Hart, Private Eye Dog," he is ostensibly the creation of animator Eliot Shag, but he exchanges wisecracks, advice, and arguments with Eliot on an equal level. Ace is also subject to the whims of Eliot's sense of humor and at the mercy of his paintbrush, leaving Ace with no resort but a plaintive, pleading cry of "Eliot!" Within the animated Dog City, Ace's allies include Eddie and police chief Rosie O'Gravy, while his chief adversaries are Bugsy Vile and his gang and the fiendish Baron Von Rottweiler.
Ace Hart served in some respects as the idealized animated alter-ego of Eliot, and appeared in puppet form in "Sick as a Dog." The puppet, and the basic character, were revised versions of Ace Yu from the original Dog City special.