The cover of Teen Titans 40 (July/Aug 72) tells the reader that the swimming masked man pictured is a friend of the Titans, who has now turned against them. Hmmm. Who could it possibly be? Thankfully, Haney, Art Saaf and Cardy do not play this out as a mystery. If you can't figure out it's Aqualad, you haven't been reading this book at all.
Mr. Jupiter brings Robin, Kid Flash, Wonder Girl, Speedy and Mal Duncan to Scotland to try to determine the truth behind the Loch Ness Monster legend. Their local guide, Gilkrankie, is certain the beastie exists, and it sure looks like it might be on the splash page, as a creature emerges to devour a fisherman's nets. Speedy shoots an arrow into the beast, which turns out to be a (probably really expensive) robot, used to steal other fisherman's catches. The cost of doing this makes one think Scottish fish must sell at absurdly high prices to make the scam worthwhile.
Even though the monster they see has been exposed as a fake, Gilkrankie insists a real one is out there, and tells a story about the Black Earl of Moray, who used magic to control the people and animals of the region, until a priest made his island sink. The Teen Titans get a photo of two underwater beings, and Gilkrankie insists they must be thralls of the Black Earl.
As the story goes on, Gilkrankie gets killed, the Earl's island rises from the sea, and the Titans get attacked by the underwater pair. Robin identifies the male as Aqualad.
And, sure enough, they find Aqualad enrolled in a school in a nearby village, and dating a blonde woman. It would be great to pretend that this was the msyterious woman from the Aqualad story a few months earlier, but it isn't. Not even close. Aqualad seems all friendly, but he is under the spell of the woman, and betrays his friends, though they capture him.
The story goes totally supernatural at this point, and the Titans probably are regretting not bringing Lilith along. The Black Earl has risen, and is taking over the minds of the townspeople, turning them into rampaging madmen. The Teen Titans prevent the Earl from killing anyone, and his island sinks, taking him and the woman, his daughter, back into the depths.
This is Aqualad's final appearance in the book. He is next seen, two years down the road, in the Aquaman back-up story in Adventure Comics.