The series follows Nai and Gareki, who join a defense organization called Circus in hopes of finding a man named Karoku.

When Nai and Gareki find themselves desperate in a hopeless predicament, they encounter none other than the country's most powerful defense organization, "Circus".

They were once human or animal, but a cell-modifying medicine given to them by Kafka slowly took over their bodies until they were fully transformed, at which point they became monsters.

[64] Funimation later added the English dub version in June 2014, alongside Date A Live, Red Data Girl and Code:Breaker.

A drama CD titled Karneval Circus was released on March 25, 2010, on the Frontier works label.