To Infinity And Beyond
GIF version which wouldn't show up:
Image source above. Not a spoiler.
This puzzle is just about the closest thing to a themeless you'll ever get from me-- which is to say, it has a theme, but the theme is just "here's a cool grid!" Everything is explained in the PDF file, but basically all you need to know is that two grids interlock to create an infinite loop.
Is this what constructors see while on 71-Down, I wonder?
Not available online, unfortunately but obviously.
(grid 1 first)
(grid 2 first)