It says: "When revealed, each player may put an in-House character of printed cost 3 or lower into play from his or her hand. Each players must choose 1 character he or she controls and return it to its owner's hand, if able."

Now it is revealed. Which one is right?

1)A puts a character into play from his hand, and returns 1 character he controls. Then B puts a character into play from his hand, and returns 1 character he controls.

2)A puts a character into play from his hand, and B does so. Then A returns 1 character he controls, and B does so.

I think the 1st one is right. Am I right?

There's a whole Rules Forum for questions like this.

The correct answer is #2. Each sentence in the effect is separate, so you resolve each one before moving on to the next. So all players put characters into play (starting with the person who revealed the plot), then all players return characters to hand (starting with the person who revealed the plot).