This book introduced “The Maggia,” so named because the Comics Code would not approve any publication that used the term “Mafia,” and the character Count Nefaria. Nefaria is introduced as a crime lord who wants to take down The Avengers because they’re bad for the crime business. He’s also a robotics expert, and he makes his own fake Avengers team and uses them to trash the team’s rep in the community. One thing he is not is super-powered. That comes later in the character’s history.
Fake versions of “real” heroes, i.e., imposters, is a favorite Stan Lee trope.
There’s also some Teen Brigade stuff, but I can’t stand that aspect of The Avengers so I’m ignoring it.
As a result of Nefaria’s efforts, the team goes underground in a “secret” base. Why do they have a secret base? It’s not explained.
During their battle with Nefaria, Wasp gets shot, which is the cliffhanger for #14.
Creators: Stan Lee, Jack Kirby
First appearances: The Maggia, Count Nefaria!
For the complete history of the MU, year by year, go here.