During the eventful summer of 2019 in Hong Kong, the Be Water Revolution formed to resist the proposed extradition of fugitives to mainland China’s courts. With its name derived from martial arts master Bruce Lee’s adage to be “formless and shapeless like water,” the movement turned out to be the city’s largest episode of contentious politics and was unique for using impromptu communication among participants and the absence of central leadership. In Be Water, Ming-sho Ho examines the dynamics of the city-wide uprising from the perspective of agency power. He seeks to understand how numerous and anonymous Hongkongers contributed to this epoch-making campaign as well as how they responded to the full-scale state repression that enveloped them. Ho praises and questions the durability of the inventive Be Water Revolution and how the activists encouraged protests spontaneously, through interpersonal networks and by voluntarily collaborating with strangers at great personal risk. Ho posits a new concept of “collective improvisation” to make sense of such a decentralized yet creative way of protesting. Be Water seeks to understand the rise and long afterlife of this movement and illustrate its efficacy. As Ho shows, these dynamics of collective improvisation have implications for contemporary protest movements around the world.