While giving children a tour of the forest, novice zoologist Yamato Kazakiri drops his childhood charm, a cube and chases after it. He then discovers the Link Cube, a huge cube structure and notices that his cube fits perfectly in the slot found in the Link Cube. He inserts the cube, a door appears, and was sucked into it.
Back in Zyuland, we learn that the Link Cube connects the "Human World" and "Zyuland" and the only way to activate the portal is by inserting all six King's Credentials into the Link Cube. Because one of the King's Credentials was lost a long time ago, and Yamato has one of it, he was accused as the one who stole it. Scared, Yamato runs back to Human World, but is chased by the four Zyuman who discovered him. When they arrived in the Human World, the Deathgaliens began their invasion.
Using the power of the King's Credentials, Yamato and the four Zyuman transform into the Doubutsu Sentai Zyuohger! Even though they have defeated Deathgalien's first attack, the Zyumans lost one of the six King's Credentials needed to open the portal back to Zyuland.
Now stuck in Human World, Yamato feels responsible to what happened and takes care of the four Zyuman and have them stay with him in his uncle's place. Sela, Leo, and Amu accepted the offer, but Tusk refuses and goes off alone. Concerned about Tusk, Yamato goes after him only to encounter a new Deathgalien.
[Source: Doubutsu Sentai Tumblr]