Following World War II, the United States recruited over 1,500 German scientists, engineers, and technicians in the classified effort known as "Operation Paperclip". Among those recruited was former HYDRA scientist Arnim Zola, who started working for S.H.I.E.L.D. in an attempt to rebuild HYDRA from inside the newly-founded peacekeeping organization.
In 1991, HYDRA learned that S.H.I.E.L.D. co-founder Howard Stark had recreated the Super Soldier Serum and tasked Vasily Karpov, a sleeper agent embedded in the Russian Armed Forces, with reviving the Winter Soldier. Karpov did as he was told, ordering the master assassin to retrieve the serum and leave no witnesses behind.
As the Winter Soldier approached the fallen vehicle, Stark crawled from the car and asked him to help Maria. He recognized the assassin as Bucky Barnes only to be repeatedly punched in the face and have his skull crushed by the Soldier's cybernetic arm.
After placing Stark's corpse back inside the car and strangling Maria to death, the Soldier proceeded to shoot at a nearby security camera before departing with the serum.
The newspapers reported on the Starks' passing, but their deaths were attributed to a simple car accident.
HYDRA used the serum to give five individuals the same capabilities as the Winter Soldier, but they proved too dangerous to be sent into the field and remained in suspended animation at a facility in Siberia.
In chronological order:
- Captain America: Civil War (flashback)
- Iron Man (mentioned)
- Iron Man 2 (mentioned)
- Captain America: The Winter Soldier (newspaper)
- Captain America: Civil War (footage)
- Avengers: Infinity War Prelude (footage)