Varys's Riddle doesn't copy an ability, it initiates it. When resolving Varys's Riddle, take the opponent's plot in your hand, and resolve its When Revealed ability as if you've just revealed that plot instead of Varys's Riddle.
If one player reveals Summer Harvest and another player reveals Varys's Riddle, then the First Player (likely the one who revealed Varys's Riddle) can choose whether the gold value of Summer Harvest is 2 or 7. (More details)
- When you resolve the when revealed ability on another player's plot card, you're considered to be the controller of that plot card while it resolves, so characters that are "immune to opponents' plot effects" (e.g. Maester Luwin, Septa Mordane, Winterfell Heart Tree) will not be protected.