Steve got the super soldier serum, went to go fight Hydra, but then somehow went evil and took over Hydra (I think Red Skull ordered him tortured, but it's been a while). One day a Hydra minion brought Steve a tortured prisoner, who Steve recognized as Bucky. Steve kept Bucky basically as his private slave from then on, where no one else was allowed to hurt Bucky but Bucky wasn't allowed to leave. Bucky would try to starve himself whenever Steve left the base (as suicide seemed to Bucky to be the only way out), so Steve ordered him force fed. There was also a bit set years later where Steve has the Hydra scientists give Bucky the serum so Bucky won't grow old and die and escape that way.
IIRC, it was told from the point of view of a hydra minion/scientist.