He wants to steal a new device--a "chloro-beam" designed to increase food production in the oceans, but that Attuma can use to turn his henchmen into super-powered giants. I'm not sure that makes sense, since the name implies it was affecting chlorophyll and not other sorts of cells, but what the heck. Comic book science is pretty all-encompassing.
So his plan is this: Attack and capture the Avengers; equip them with slave collars to force them to attack Namor; then, while they are all hopefully destroying each other, he'll attack the sea base at which the chloro-beam is being tested and steal it. But Namor isn't where Attuma thought he would be and the Avengers end up attacking Dr. Doom instead.
Attuma attaches the slave collars to his captives and sends them after Namor, who at the time has was residing at Hydrobase, a floating U.S. government facility. But the story arc taking place in Super Villain Team Up has put Namor in Latvaria, while Dr. Doom is at Hyrobase.
The Avengers don't fight well while enslaved and are captured. In the meantime, Beast has recruited two heroes who had been involved in the previous storyline--the recently resurrected Wonder Man and the World War II-era speedster with the unfortunate name of the Whizzer. (Seriously, even in the 1940s, how could that name NOT have made 8-year-old boys giggle?)
The art is by George Perez, whose work is always exceptional but who really seems to shine particularly bright on team books. Gerry Conway wrote the script and handles the twists and turns quite expertly. I really do enjoy the way he took the characters down different roads to get them all to the same place.
I grant that events taking place in the team up book cross over with this story line, but all the same I dislike this. It forces you to buy an issue of a comic you don't necessarily want to get the entire story. I understand that comic books are a business and must be marketed effectively. And I love it when characters from different books cross over with each other. That can be a lot of fun and its a legitimate way to get readers interested in other books.
But buying a particular book should always be a choice. It should never be forced upon you. Crossing stories throughout different books has become annoyingly common over the last couple of decades. In 1976, it was fortunately more rare. And comics were only 30 cents at the time, so it wasn't has painful a financial experience buying one as it is now. But was still an annoying thing to have to do. Besides, at the time, comic book stores were still rare. You got comics off the rack at your local 7-11, so missing an issue was all too real a possibility.
Darn you, Marvel Comics. See what you did to me.