Avengers Vol. 1 (1963-1996, 2004) # 4
1st Silver Age appearances of Captain America (Steve Rogers), and Bucky Barnes. Origin of Cap & Bucky retold.
1st app of Baron Zemo (Heinrich Zemo) is as a flashback.
"Captain America Joins... The Avengers!" – (23-page story) – After escaping the Avengers previously, the Sub-Mariner encounters Inuit worshiping a man in a block of ice in the north sea. Angrily, he casts the frozen man into the ocean. The Avengers discover the body, which had been thawed by warmer waters. It turns out to be the hero of WWII, Captain America. His body had been frozen and preserved after a failed mission near the end of the war. The Avengers then attend a press conference where they are turned to stone by a creature with a mysterious ray.
Owing his life to the Avengers, Captain America tracks down the alien who explains that his ship crashed in Earth's ocean a long time ago and that Namor promised to free his ship in exchange for turning the Avengers to stone. Captain America makes it restore the Avengers back to normal. In return the Avengers try to free the alien's craft from the water and the Sub-Mariner attacks. After fighting him off, they finish freeing the craft and the alien departs. Captain America is then recruited as an Avenger.