- "You know, for being big, lopsided crabs with way too many eyeballs, these guys are really all right."
- "We are the Mercora. We are immigrants to this planet. [...] This is our world now. Our original planet was destroyed when our sun was drawn toward a black hole. We are all that is left of the Mercora. And we cannot leave this planet. Not that we would ever wish to. It is wonderful. Wonderful. And it will be our home forever."
- ―The Mercora to the Animorphs
The Animorphs first meet the Mercora as they land on the force field surrounding the Mercora base in a valley. After introducing themselves and realizing that the Animorphs are not a threat, they allow the Animorphs to live inside the colony, giving them a space to live as well as supplying the Animorphs with broccoli, and a rat for Tobias. Ax, who is communicating with the Mercora scientists and using their computers, comes to the conclusion that they must steal a nuclear explosive from the Nesk base in order to close the Sario Rip and return to their own time.
The Mercora use one of their two ships to fly the Animorphs to the border of the Nesk base, giving Ax a thought-speak communicator. After stealing the nuclear weapon, Ax calls the Mercora for help, who sacrifice one of their ships in order to destroy a Nesk ship. The Mercora rip out some of their limbs as tribute to the Mercora and the Nesk who perished. However, the Nesk evacuate the planet following their loss and in spite, redirect a nearby comet's trajectory to impact the Mercora colony. The Mercora wish to use their last remaining ship to attach the nuclear weapon the Animorphs stole to the comet, destroying its head before it strikes, thereby preventing their extinction, although the Animorphs would be trapped in the Cretaceous era forever. Tobias privately orders Ax to dismantle the weapon, and then presents it to the Mercora. Tobias then informs the other Animorphs that the comet that is hurtling is the very comet that wiped out the dinosaurs, and since no scientists ever found a Mercora fossil, the Mercora would die out as well. The Animorphs then fly to the ocean and await the comet's impact, which incinerates and kills all the Mercora while the Animorphs are transported back to their own time.