1 Monty: Okay then... here goes... {rolls die}
1 [sound]: Clatter!
1 Monty: The die is cast.
2 Prof. Jones: Six! Well done, Junior! And look, something else has popped out of the slot.
3 Monty: It's a bigger cube, with nine squares of different colours on each face. I count six different colours all together...
4 Minnesota Jones: There's no going back now. We've crossed the Rubicon.

I suspect most people reading this comic will get all the jokes and references here. Sometimes I'm a little reluctant to actually explain what's going on, since I run the risk of talking down to people who already know. If you did get the classical references, there's really no need to read any more of this annotation.

If you're still with me, we'll begin with "The die is cast", which is a common English translation of the Latin sentence "alea iacta est", said to have been uttered by Julius Caesar on 10 January, 49 BC. The occasion was his leading his army across the Rubicon River into Roman Italy, precipitating a war that eventually led to Caesar assuming the dictatorship of the Roman Republic. With this act, he set in motion a course of events from which there could be no going back. The "die is cast" comment in this context means that he has irrevocably cast his lot to Fate, and that he must now accept whatever outcome awaits.

From this comes the phrase "crossing the Rubicon", which is a metaphor for passing a point of no return, from which there is no going back.

The last piece of the puzzle is provided by Hungarian inventor Ernő Rubik, best known for his eponymous Rubik's Cube.

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2017-12-03 Rerun commentary: The die shown in this comic is made of white LEGO bricks: a 1×1 tile attached to a 1×1 plate, which I placed in the set and photographed. I added the black pips (i.e. the spot making up the numbers on each face) in Photoshop. They look suspiciously like they're moving around slightly between the different panels, which is a good indication of that.