The universe has died, but Gordon hasn't.
Having survived his own spontaneous human combustion, Gordon's living head now resides inside his girlfriend Ardel's handbag, grumbling because his right foot (which is in a plastic container elsewhere in the house) has an itch.
Looking outside, Ardel and Gordon see a world where weekends are reported missing, superintelligent murder missiles scream through the sky, and ghostly alien mushrooms borrow human brains for an evening. Reality is falling apart, and Gordon is the only person capable of surviving a total timeline collapse, whatever one of those is.
But he's no mere passenger. Every time Gordon makes a decision, no matter how minor, reality splits in two. He's somehow embedded in the fabric of spacetime itself, like a sentient stain.
And now that stain must save the universe.
Book 2 of the WHO SOMETHING THE HUMANS? series (though you can read them in any order).