Zoids: Chaotic Century is the first of four anime series based on the Zoids range of mecha model kits produced by TOMY. Although the first series to be produced for the franchise in Japan, Chaotic Century was the second series to be dubbed and aired in Western nations, following Zoids: New Century Zero.
Like all Zoids' series, Chaotic Century is set on the world of Zi, located on the opposite side of the Milky Way galaxy to Earth. The world of Zi is inhabited by large mechanical lifeforms known as Zoids, which have been modified and harnessed by stranded human colonists as tools and weapons of warfare.
The setting of Chaotic Century is a few years after the most recent war between the Helic Republic and the Guylos Empire. The two nations are currently observing a ceasefire, although ambushes and minor assaults are initiated by both sides, and tensions are high after it is revealed that one or both sides may have access to the ancient technology of the Ancient Zoidians.
The series follows the story of Van Flyheight, as he helps his companion, Fiona, learn about her past. The pair become caught up in the beginnings of a new war, and direct every effort towards stopping it before it again engulfs the two nations.
Later, the four end up at one the Republic's army bases, and end up having to help stop an invasion by the Empire orchestrated by Prozen. After that, Van, Fiona, Irvine and Moonbay preferred to stay out of the war. However, Van ends getting involved somewhat when he meets the Guylos ace pilot Raven, whom he quickly holds a grudge against after seeing Raven's brutal nature, but proves no match for him their first battle. Van later challenges Raven again, loses, and Zeke suffers near-fatal damage, but Van manages to save him. During this time the Imperial army is marching farther and farther in Republican territory but are eventually stopped by the Republic's Gojulas. Van and the others eventually reach the Republic's capital in hopes of finding information about Zoid Eve, but only end up getting caught up in the Empire's assault on the city. During the battle Van is finally able to defeat Raven.
The war is stopped when the Empire's crown prince Rudolph orders a ceasefire, but Prozen refuses to give up on conquering the Republic, and so attempts to assassinate the prince, but his attempt is foiled by the bandits Ross and Viola who kidnap the prince in attempt to gain money. During this time, Prozen finds the remains of the Death Saurer, attempts to revive it, and also creates the Geno Saurer for Raven to use. Raven uses the Geno Saurer to destroy Van's Shield Liger, but was unaware that Zeke and Fiona revived the Shield Liger as the Blade Liger. During this time it's revealed that Fiona is actually an ancient Zoidian that existed before humans came to the planet Zi. Rudoulf joins Van and his friends who try to take him to the Empire's capital Guygalos to stop Prozen from taking over the Empire and restarting the war. During this time, parts of Imperial army loyal to Prozen and some that were manipulated into fighting them try to stop them. Eventually, Van faces Raven again and after an extremely difficult battle destroys the Geno Saurer, but Raven survives thanks to his organoid Shadow. Although Rudolph is returned to the capital, Prozen attempts to use the revived Death Saurer to take control, which goes on a rampage and almost destroys the capital and joint forces the Empire and Republic that are powerless to stop it, but Van is able to destroy the Death Saurer and appears to kill Prozen in the process. Rudolph is then crowned the emperor and peace finally starts to settle between the Empire and Republic, but Van and Fiona don't take part in celebration and instead to leave to find Zoid Eve.