It's the middle-grade team-up of the century as the heroes of New York Times bestselling author Ben Hatke join forces to save the world in Mighty Jack and Zita the Spacegirl. Jack and Lilly are no strangers to heroics. They've befriended dragons, battled giants, and even earned the loyalty of a goblin army. So when they meet Zita the Spacegirl, fresh from her interplanetary travels and seeking their help to face a new threat, they're more than ready for another adventure. But the danger growing just outside the door to their world is greater than anything the new friends could have imagined. An army of giants and screeds stands ready to lay siege to Earth, determined to put the age of humans to an end. With the gate between worlds growing weaker and time running out, can the heroes come together to save their world from their greatest enemy yet?