Once deemed a far-fetched vision, emerging deployments of massive satellite constellations will soon offer true global coverage. When these constellations overlay the evolving terrestrial networks, a new hybrid continuum is formed which has the potential to provide uninterrupted coverage. In this paper, we provide an analytic framework for the uplink coverage probability in hybrid satellite-terrestrial networks. The framework extends well-developed terrestrial models that utilize tools from stochastic geometry by incorporating an additional layer that fits the emerging next generation satellite-terrestrial networks. The paper captures the impact of both (i) the constellation size, and (ii) the terrestrial base station density on the coverage of the uplink traffic which is dominant in applications relying on wireless sensor networks, such as the Internet of Things. This framework provides insights that guide the design of hybrid network infrastructures for a desired quality of service.