Professor Penrod sends Danny and Lin a mysterious package filled with biting, scratching microsaurs hungry enough to chew through walls, along with a very large microsaur egg.

"Danny and Lin have gotten pretty good at taking are of tiny dinosaurs so when Lin's mom asks them to babysit Lin's little sister, ChuChu, they're sure they've got it covered. After all, what better place to spend an after noon than hanging out in the Fruity Stars Lab and playing with the Microsaurs? They will be the best babysitters ever! But when the biggest Microsaur they've ever seen starts a stego stampede. Danny and Lin realize they are up for...

"As the first official members of the International Microsaur Protection Agency, Danny and Lin thought that they were prepared for anything. They've flown with Tiny-Dactyls, herded Tiny-Stegos, and wrangled Tiny-saurus Rexes. But when the Professor's newly upgraded Expand-o-matic messes up, they end up facing their biggest problem yet... a full-sized Triceratops let loose in the middle of the town's yearly Tomato Festival! Will Danny and Lin be able...