Only someone heartless would want to end the world.

The Egyptian god Set is that someone; he's heartless. Literally, he has no heart. His mentor, Ptah, ripped out Set's ticker to use as a part in the Grand Machine that runs the universe. Set is a god, so missing a vital organ isn't going to kill him. But the heart belongs to Set, and he wants it back, even if its removal from the Grand Machine means the end of the universe.

Medusa does have a heart, and it's breaking-not that Modifixeus has noticed. As the world starts to slip, she puts aside her unrequited affection to help save it. Only a monster can stop a monster after all. And where she has to go live many monsters. She might not have snakes for hair anymore, but that hasn't changed how she sees herself.

Mod has no idea what all of this heart talk is about. But he's certain he can fix the Grand Machine-he's pretty sure anyway-as long as he can make it without the security of his brother. Medusa seems to have confidence in his abilities, even if she's acting really weird. It will all be fine, right? Mod, for the first time in his life, is suddenly not so sure.

Medusa and Mod just might, possibly, be able to dodge all the monsters barring their path and find a way to fix the almost unrepairable Grand Machine before time runs out, if they can just manage to work out their personal issues first.

The world is doomed.