The Teen Titans get their first recurring villain, the Mad Mod, in the Haney and Cardy story from Teen Titans 7 (Jan/Feb 67).
The story centres on a British rock musician, Holly Hip. Wonder Girl is a huge fan of the guy, and is thrilled when the government asks her, Robin, Aqualad and Kid Flash to accompany Holly on an American tour. He is suspected of being connected to a smuggling ring, which the teens are sure cannot be the case.
At first the evidence does not look good for Holly, but further investigation proves that the real mastermind is the Mad Mod, the guy who designs Holly's clothes. His stage costumes are being used to carry rare chemicals, but without Holly's knowledge.
There is a lot of action as the Titans try to take down the Mad Mod and his gang, and the designer has variety of useful outfits of his own, as the chase goes from sea to land to air.
A lot of what makes this work is Cardy's art, and the overall lighthearted nature of Teen Titans stories from the period. It's really a very simple tale at the core of it.
The Mad Mod returns the following year.