In a standard solitaire game of Dominion, it's the beginning of your turn. You have no cards on any mats, no Durations in play, and no active Princes. There are no Events on the board.
You have somehow reached the following state: none of the cards in your hand or deck cost more than zero, except for one King's Court and one Forge.
I suppose it doesn't count if we gain other cards too, like Border Village instead of Gold for Duchies?
I like this idea but it seems to go against the terms of the puzzle.
I'll just end it so that you have time to process your anger and disappointment.
Using Madman for draw and Actions, and Spoils for coin:
Play Trusty Steed for Silver. Madman to draw these and the Spoils you'll need.
Forge Silver into Black Market
Draw and King's Court the Black Market
Buy Messenger, gain Fortress, Border Village, gain Fortress, buy Stonemason and gain King's Court and Forge.
Draw all cards: for Province, Kings Court Forge on the two Fortresses. For Duchies, play Princess first, then Kings Court on Forge and trash one Fortress and one Silver three times.
But can we get to Colony?
We have 3 Silvers, King's Court, Forge, Stonemason, 2 Fortresses, Messenger, Border Village, and an arbitrary amount of 0 cost cards, so long as they can all be drawn with Madman.
Using 7 Madmen before, we can draw up to 4*2^7=512 cards. That should be sufficient.
Overall, we have 10 Madmen, Trusty Steed, and Border Village for 12 possible actions this turn, 3 of which have already been used. So we have 9 left.
Action 1: Stonemason Fortress for 2 Develops. Play Madman to draw them.
Actions 2-3: Develop Fortress into Develop and Band of Misfits twice. Play Madman to draw them.
Actions 4-6: Develop Fortress into Develop and Band of Misfits 3 times, playing Band of Misfits as Develop.
Action 7: Play Band of Misfits as Trader, trashing Messenger for 4 Silver. Play Madman to draw everything.
Action 8: King's Court Forge, trashing 2 Develops and Band of Misfits for Colony, and trashing 2 Silvers and Band of Misfits for Colony twice.