Bonk Choy finds an underground ruin in the forest of Plant Town and decides to explore with his friends. They discover amazing things there which tell of an ancient secret such as wall carvings of dinosaur, a huge golden horn and strange inscriptions. At the time, Dr Zomboss drills his way into the ruin out of the blue! Encountering such emergency, Peashooter blows the golden horn whereby the sound sends the Plants and Zombies back to the Jurassic. There, a kind Heterodontosaurus saves the Plants from being caught by Monolophosaurus. The Plants also befriend Gasosaurus and discover thesecrets of the underground ruin from Shunosaurus. To fund the way home, the Plants join forces with Massospondylus and Huayangosaurus to attack Monolophosaurusin the golden palace.