The Winter Soldier's mission is nearly complete. Howard and Maria Stark are dead, leaving him to dispatch their four-year-old child. One quick twist of the neck is all it will take, but the Soldier finds he cannot do it.So instead of killing Tony Stark, the Winter Soldier takes him away to raise as his own.
Be aware - the beginning of this one is quite dark, if you're sensitive to child trauma, but it does get lighter with time, coming to be endearing and uplifting as Bucky slowly begins to return to himself and Tony adjusts.
Good - Child Tony is remarkably well aged - brilliant and precocious for a four year old but nonetheless four - and that characterization becomes only more impressive as Tony ages. It's fabulously done and adorable.