In Parts 4 and 5, the MacGuffin is the gold. But cleverly subverts the heist genre: they don't just want to steal the gold; they want to melt it down and send it back to the Bank of Spain coated in silicon so it looks real, while the real gold is buried under a cabin in the woods. It’s a logistical masterpiece.
In the final heist, the Professor develops a method to melt gold using a chemical alloy that turns it "yellow" but fake. He then constructs an underground forge in a remote cabin. The logistics of moving 90 tons of gold bricks across a war-torn Madrid require the most complex planning in the series. Watch closely; you might miss the chemical formula. Money Heist Az