Integrated Ocean Drilling Program (IODP) Expedition 314 is part of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), an ambitious, coordinated, multiplatform and multiexpedition drilling project designed to investigate fault mechanics and seismogenesis along subduction megathrusts through direct sampling, in situ measurements, and long-term monitoring, in conjunction with allied laboratory and numerical modeling studies. During Expedition 314, our primary goals are to obtain a comprehensive suite of geophysical logs and other downhole measurements at six sites, using state-of-the-art logging-while-drilling (LWD) technology. All six of these sites are slated for continuous core sampling during subsequent IODP NanTroSEIZE expeditions. These sites include sampling of the incoming plate trench sediments, the frontal thrust and toe of the accretionary prism, forearc basin deposits, a major out-of-sequence thrust system (the "megasplay" fault), and highly-deformed rocks of the interior of the accretionary prism. The principal goals of the LWD program are to document in situ physical properties; stratigraphic and structural features; sonic to seismic scale velocity data for core-log-seismic integration; and stress, pore pressure, and hydrological parameters. Depth objectives for this expedition range from approximately 600 to 1400 meters below seafloor. Logging plans are not final, pending negotiation with logging service companies, but we anticipate that the minimum measurements will include natural gamma radiation, azimuthal gamma ray density, neutron porosity, full waveform sonic velocity, azimuthal resistivity imaging, zero-offset vertical seismic profile, ultrasonic caliper, and annular fluid pressure. In this Scientific Prospectus we present the scientific background and objectives, the drilling and logging operations designed to achieve them, a contingency plan strategy, the currently understood risks and steps taken to mitigate them, and the coordinated Stage 1 plan for sharing samples and data.