The author developed a mobile, mixed-reality version of capture the flag, a popular genre of computer game. Our CTF game is novel in three ways. First, the smart phone is the main interface. Using the smart phone, players physically role-play virtual characters who try to capture enemy flags by traversing different landscapes. This approach creates a direct, real-time linkage between the real and virtual worlds. Second, players can move freely in the real world over a wide area while maintaining seamless real-time networked contact with other players in both the real and virtual worlds. Third, CTF explores novel tangible aspects of human physical movement and perception, both in the real-world playing environment and in interaction with the virtual world.