The Orville crew discuss the Kitan's breakup, Bortus' digestive system, and Isaac's sexual curiosity. The bridge summons them all for a distress call.
They detect a Krill battle cruiser above a base, Castro-4, that has been attacked several times. Mercer orders the Krill ship to stop firing so it stops and turns on the Orville instead.
The Orville engages in a battle but is badly out-powered and Mercer orders Malloy to take the Orville down towards the upper atmosphere of the planet where he orders Bortus to drop all torpedoes on the Krill ship which destroys the cruiser.
Mercer sends the medical team down to the planet. Grayson spots a Krill shuttle floating in the wreckage of the cruiser. Admiral Ozawa joins the Orville and, instead of taking the shuttle back for dissection and examination, she order Mercer and Malloy to go undercover, infiltrate a Krill ship and obtain a copy of the Krill bible, the Anhkana.
When the shuttle is ready to depart, Mercer is marched into the shuttle bay by a Krill who turns out to be Malloy disguised as the Krill. Grayson expresses concern for Malloy's well-being.
On the shuttle, Mercer and Malloy try to come up with Krill names for their undercover assignment. Approached by the Yicar, a Krill destroyer, they activate their disguises and dock their shuttle on the ship. When they board, they are taken to the captain and head priest, Sazeron. Telling the Krill that they are the sole survivors of the Union battle, they join the rest of the Yicar crew in religious services, worshipping their god, Avis.
Entering the services space, they meet a Krill woman, Teleya, whose brother was on the Kakon. The priest brings out a human head and then stabs it several times as part of their worship. After the service ends, they sneak back in to take pictures of the Anhkana. Sazeron interrupts them and they make up a cover story and get away.
They sneak back into the Chapel again after hours. Meanwhile, Sazeron requests that a guard be put on the chapel. As the guard approaches, the holographic emitters fail, leaving Mercer and Malloy undisguised.
Managing to escape back to their quarters, they fix their emitters and track the interference to a massive bomb being built in the lowers decks of the ship. Malloy wants to get out but Mercer wants to learn what the plans are for the bomb. They track down the female Krill and she reveals the bomb is capable of wiping out all life on a continent and its target is Rana-3, a farming community.
Mercer thinks they can overload the bomb using their emitters and Malloy points out that their mission was in the interests of peace but now they are planning to blow up everyone on board the Yicar. Going out to find tools, Teleya asks them to speak to her trainees who turn out to be children. Leaving the classroom, Mercer refuses to kill children
One of the children follows them back to their quarters and his questions inspire Mercer to use UV to incapacitate the Krill.
Mercer goes back to the classroom while Malloy sets the trap. Malloy gets caught by Sazeron and the crew is put on alert for Mercer. He convinces Teleya to keep the kids in the classroom. Malloy is interrogated by the captain who stabs him in his brand new leg.
The captain launches the bomb just as the UV timer counts down and burns all the Krill not in the classroom to a crisp and Malloy gets a sunburn. Malloy fires the ship's weapons and destroys the bomb before it can land.
Returning to the Orville, Mercer visits Teleya in the sick bay and she points out that sparing the children means that they will grow to be his enemy since he killed all their parents.