Freefall

"But Zola was working for SHIELD, or at least it was still called SHIELD, and Dad was many things, but no one would’ve called him trusting. If they managed to recreate the serum, they would’ve had to pry it from his cold, dead –”
Tony froze. Sam winced.
“Sam,” Tony turned towards him, his face eerily blank, “How did HYDRA get its hands on the supersoldier serum?”
(Or: Sam very nearly catches Rhodey. Tony learns the truth several hours before a bunker in Siberia. Zemo remembers the trigger words. And Ross is losing patience fast.)