Things to know:

James Buchanan "Bucky" Barnes is the best friend of Steve Rogers A.K.A. Captain America, and both are born in the 1910s. During World War II, Bucky was captured by the enemy, Hydra, and for the next 70 years, he was experimented on, tortured, brainwashed into being an amnesiac assassin codenamed, The Winter Soldier. Between missions, he was cryonically frozen to keep him at his prime for when they needed use for him.

In 2014, the Captain America: The Winter Soldier movie, near the end of Hydra's plan to cleanse the world of those who could oppose them, the Winter Soldier was ordered to stop Captain America, who just found out his best friend was alive. Before Bucky could kill Steve, he breaks out of Hydra's programming and disappears from everyone's radar.

And maybe that his left arm's been replaced with a shiny robotic one.