Don't Do Anything Rash

A flashback opens on an enslaved alien race, the Relonians, whose revolt against their Viltrumite overseers triggers a deadly explosion. In Emperor Argall's chamber, the leadership debates the response. Kregg calls for a mass culling while Thaedus argues that the empire's cruelty bred the uprising, and Regent Thragg sides with Argall's belief in a natural order that the weak must be forced to accept. Argall orders a quarter of the Relonians killed and hints the whole species could follow, silencing Thaedus. Soon after, Thaedus assassinates the emperor, clearing the way for Thragg to rise as Grand Regent, launch the Great Purge, and reshape the Viltrumite people through fear.

In the present, Thaedus briefs the Coalition on Thragg's position and the scattered Viltrumites who have not yet returned home. Mark records what may be a farewell message for Eve, and the crew shares an uneasy meal before their warship reaches the corpse-strewn Viltrumite Ring, where Thragg and his survivors lie in wait.

The ambush explodes into open warfare. Thaedus, Battle Beast, Nolan, Allen, Tech Jacket, Mark, and Oliver scatter against Thragg's faction of Thula, Kregg, Lucan, Anissa, and others. Space Racer turns the tide by unleashing a pack of Rognarrs that shred much of the enemy force. Thragg drags Nolan aside to argue over the Viltrumite future and the Scourge Virus Thaedus engineered, then re-enters the fray. He tears the arm and jaw from Oliver, and when Mark strikes him with everything he has, Thragg absorbs the blow with a demonic grin and calls it unimpressive.

Seeing no other option, the heroes commit to destroying the planet itself. Space Racer's Infinity Ray, driven home by Thaedus, Nolan, and Mark, ignites Viltrum's core and obliterates the world. A shattered Thragg refuses surrender, rips off Thaedus' head, crushes it, and beats Nolan mercilessly. He drives a fist through Nolan's chest and starts to gouge Mark's skull, yet at the last instant he releases the boy and withdraws with his remaining warriors rather than finish him.

The Coalition warship recovers a broken Mark and Nolan and carries them in for treatment. Weeks later Mark wakes to learn that Allen now leads the Coalition following Thaedus' death, while Nolan keeps vigil over a comatose Oliver being fitted with a mechanical arm and jaw. Puzzling over why Thragg spared him, Mark realizes the surviving Viltrumites have vanished from every nearby sector and deduces their likely destination: Earth. The seventh episode of the fourth season, and the thirty-first overall, its title echoes a warning Cecil once gave Mark in the comics, and it finally grants Emperor Argall a full voiced appearance while dramatizing Thragg's rise to power.