Today's "white space" model of spectrum sharing applied in the UHF TV band allows channels that are not being used regionally by a TV broadcaster to be re-purposed for unlicensed-style secondary access in 24 hour increments. Unfortunately, populated areas have few unused channels for white space usage. Nonetheless, from the UHF TV viewer's perspective, Nielsen data show severe under-utilization of this spectrum, with vast regions that are in range of TV transmitters having no active TV receivers on multiple channels even at peak TV viewing times. In this paper, we present the design, implementation, and experimental evaluation of WATCH (WiFi in Active TV CHannels), the first system to enable secondary WiFi transmission even in the presence of kilowatt-scale TV transmitters, while simultaneously protecting TV receivers when they are active. To protect active TV receivers, WATCH includes a smartphone-based TV remote or an Internet-connected TV to inform the WATCH controller of TV receivers' spatial-spectral requirements. To enable WiFi transmission in UHF bands, we design WATCH-IC (Interference Cancellation) and CAT (Constructive Addition Transmission) to (i) exploit the unique environment of asynchronous WiFi transmission in the presence of a strong streaming interferer, and (ii) require no coordination with legacy TV transmitters. With FCC permission to test our implementation, we show that WATCH can provide at least 6 times the total achievable rate to 4 watt secondary devices compared to current TV white space systems, while limiting the increase in TV channel switching time to less than 5%.