Captain America calls the four Avengers to Dr Doom's American castle, abandoned since Fantastic Four #5. He wants to use Doom's time machine to go back to see if Bucky really died in WWII. Black Panther, Goliath and Hawkeye elect to go with him. Wasp stays behind to (wo)man the controls.
As planned they arrive on the fateful day invisible and intangible. The idea is to observe only. Reed Richards warned them that two versions of Cap existing in the same time and place could be catastrophic.
They see Cap and Bucky trying to stop Baron Zemo stealing an experimental drone plane. Zemo grows a giant humanoid from a small 'seed' to protect him, and Cap and Bucky are KO'd by Zemo and his minion.
Zemo then covers the heroes' costumes with GI uniforms, so they won't die as his hated enemies. The humanoid ties them to the drone, which Zemo is going to send to Berlin.
At this point in the future Wasp experiences a sudden drowsiness, and 'accidentally' pushes a button which makes the Avengers materialise in WWII.
Cap attacks Zemo, and a stray shot from Zemo's gun grows another humanoid. Both giants attack the Avengers and some GI guards who come to investigate the noise.
But then the Avengers start to dematerialise again. Cap's last act is to send his shield to cut Steve and Bucky free, just as they are regaining consciousness. The once-more-immaterial Avengers see Zemo launch the drone, Steve and Bucky chase after it on a motorbike. Both leap for it but only Bucky latches on. The drone is booby-trapped. Steve falls into the sea, and the drone explodes with Bucky on it.
The Avengers return to their own time. Cap is now convinced Bucky is dead. Wasp thinks her little sleep didn't do any harm. But Annual #2 which follows immediately afterwards will show that she's wrong.