The Longest Day in Chang'an is produced jointly by Youku, Beijing Weying Technology Co., Ltd, Liu Bai Entertainment and Yuyue Film Co., Ltd.

The series follows the characters Zhang Xiaojing and Li Bi, and their efforts to foil a terrorist attack on Chang'an, the capital of the Tang dynasty (618–907).

After retiring, he joined the local security of the city of Chang'an, the heart of the empire, but he was imprisoned and sentenced to death after an incident in which Zhang killed 34 members of a gang that murdered his former Army commander Wen, and then killed his current commanding officer.

Zhang is unexpectedly reprieved when plans for a major rebellion are uncovered, involving a terrorist cell, the Wolf Squad, which has slipped into Chang'an before the Lantern Festival, one of the most spectacular days in the ancient Chinese calendar.

It was released in North America through video streaming website including Viki, Amazon and YouTube.