Q: Does Plan C from the Get the MacGuffin expansion use your action when you put it in your hand on your turn, or can I then use my turn action to put it back into play on the table?

A: Yes, putting it back in your hand is your action for the turn. Thus it alternates between being in your hand and in play. This is one of the ways it differs from the MacGuffin and Backup MacGuffin, which can be picked up and replayed as your turn action.

Note, however, that, unlike the MacGuffins, Plan C’s power (to not diminish your hand) is not dependent on it being your only remaining card, so in some ways it’s more like the Crown, in that it is useful earlier in the game, before there is a MacGuffin in play.