Fics like Four times an Avenger met Bucky and didn't know who he was, and one time they were all introduced.

What it says on the tin. In which Tony is kidnapped, Clint is a fangirl, Bruce sees echoes of the past and Thor is Thor.

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Let's Fall in Love by Desdemon (Captain America (Movies), The Avengers (Marvel Movies), more)
Mistake on the Part of Nature by idiopathicsmile (Captain America (Movies))
Aquired Taste by 74days (Captain America (Movies))
Took Me By Surprise and Then by thehoyden (X-Men: First Class (2011) - Fandom, The Avengers (2012))
Sixteen Fun Facts about the Avengers by miscellea (Marvel (Movies), The Avengers (2012))
Slide To Answer by relenafanel (Captain America (Movies))
Friends Don't Let Friends by theorytale (The Avengers (2012))
With Friends Like These by prettybirdy979 (Daredevil (TV), Captain America (Movies), more)
Forgetting It's There by spinstitcher (Amazing Spider-Man (2012), The Avengers (2012), more)
All The Leaves Are Brown (And the sky is gray) by AvocadoLove (Captain America (Movies), The Avengers (Marvel Movies), more)
Photo Booth by copperbadge (Marvel, Captain America (Movies))
chain reaction but (you’re holding the fuse) by notcaycepollard (Captain America (Movies))
Fight or Flyte by theorytale (The Avengers (2012))
do i tell you i love you or not (cause i can't really guess what you want) by notcaycepollard (Captain America (Movies))
Come To Morning by orphan_account (Captain America (Movies))
Number Cruncher by cauldronofdoom (The Avengers (2012))
Your Lack Of An Answer Is Kind Of An Answer: Four Questions Natasha Asked Steve Rogers, And One Time Bucky Barnes Answered by Speranza (Captain America (Movies), Marvel Cinematic Universe)
Save a Horse, Ride a Captain by galwednesday (Captain America (Movies), The Avengers (Marvel Movies))
The Care and Feeding of Traumatized Ex-Assassins by Sholio (Captain America (Movies))
elioenai (The Avengers (2012), His Dark Materials - Philip Pullman)