ICE GIANTS ARE THE ONLY MAJOR CATEGORY OF SOLAR SYSTEM OBJECT NEVER TO HAVE HAD A DEDICATED MISSION AND REPRESENT ONE OF THE LARGEST GROUPS OF DETECTED EXOPLANETS. THE COMPLEX NEPTUNIAN SYSTEM PROVIDES THE UNIQUE OPPORTUNITY TO SIMULTANEOUSLY STUDY AN ICE GIANT WORLD AND A LARGE CAPTURED SATELLITE (TRITON) THAT MIGHT BE HOST TO A SUB-SURFACE OCEAN. SUCH A MISSION REPRESENTS A WEALTH OF CROSSCUTTING SCIENCE OBJECTIVES THAT CAN IDEALLY BE ADDRESSED WITH A FLAGSHIP MISSION. ICE GIANTS CHALLENGE OUR UNDERSTANDING OF PLANETARY FORMATION EVOLUTION AND PHYSICS. FOR EXAMPLE MODELS SUGGEST THAT THEY HAVE A NARROW TIME WINDOW FOR FORMATION: THEIR ROCK/ICE CORES MUST BECOME LARGE ENOUGH TO GRAVITATIONALLY TRAP HYDROGEN AND HELIUM GAS JUST AS THE SOLAR NEBULA IS BEING DISSIPATED BY THE EARLY SUN. FORMING EARLIER WOULD CAUSE THEM TO TRAP LARGE AMOUNTS OF GAS AND BECOME LIKE JUPITER AND FORMING LATER WOULD NOT ALLOW THEM TO TRAP THE GAS THEY HAVE (PERHAPS 10% OF THEIR TOTAL MASS). BUT IF THEIR FORMATION REQUIRES SUCH SPECIAL TIMING WHY ARE THEY SO COMMON? TRITON S YOUTHFUL SURFACE GEOLOGY AND THE SOUTH POLAR PLUMES THAT WERE IMAGED BY VOYAGER 2 SUGGEST THAT IT MAY BE AN OCEAN WORLD POSSESSING EITHER A GLOBAL OCEAN OR OTHER LIQUID RESERVOIRS OF SOME FORM. THE EXTENT TO WHICH CRYOVOLCANISM WHICH IS INCLUSIVE OF PLUME ACTIVITY WITH AN ENDOGENIC ORIGIN HAS RECENTLY OCCURRED ON TRITON COULD ALSO REVEAL MUCH ABOUT THE ICY MOON S INTERNAL HEAT BUDGET AND HOW IT HAS EVOLVED OVER TIME. A CAREFULLY PLANNED NEPTUNE ORBIT WOULD ENABLE OUR FIRST REAL UNDERSTANDING OF NEPTUNE S STRUCTURE AND TRITON S GEOSCIENCE. THE MULTIPLE VANTAGE POINTS AND DURATION OF THE TOUR WOULD ALLOW OBSERVING OF THE COMPONENTS OF THE NEPTUNE-TRITON SYSTEM UNDER VARYING GEOMETRIES AND CONDITIONS AS WELL AS ALLOWING FOR SERENDIPITOUS DISCOVERY AND FOLLOW-UP. ADDITIONAL MISSION ELEMENTS WE PROPOSE TO CONSIDER ARE: 1) A NEPTUNE ATMOSPHERIC PROBE TO MEASURE HEAVY NOBLE GASES ISOTOPIC RATIOS AND THE BULK ABUNDANCE OF CERTAIN SPECIES AND 2) A TRITON LANDER TO PROVIDE SYNERGISTIC OBSERVATIONS FROM THE GEOLOGIC OUTCROP SCALE TO PROVIDE GROUND TRUTH FOR THE ORBITER S OBSERVATIONS. GEOCHEMICAL LABORATORIES ON THE LANDER COULD PERMIT CORRELATING SURFACE COMPOSITION WITH MULTI- OR HYPERSPECTRAL IMAGING BY THE ORBITER. PUBLIC ENGAGEMENT WOULD BENEFIT FROM INSPIRING VIEWS FROM THE SURFACE PERHAPS WITH NEPTUNE APPEARING IN THE SKY AND VIEWS OF WEATHER FROM TRITON S SURFACE WOULD COMPLEMENT DIFFERENT PERSPECTIVES FROM THE ORBITER. A MISSION TO THE NEPTUNE-TRITON SYSTEM WOULD ALLOW NEXT-GENERATION COMPARATIVE STUDIES OF KUIPER BELT OBJECTS AND ADVANCE OCEAN WORLDS HELIOPHYSICS AND EXOPLANET OBJECTIVES. WE HAVE IDENTIFIED A TALENTED DIVERSE AND MULTI-DISCIPLINARY TEAM TO CARRY OUT THE CONCEPT STUDY LEVERAGING THE FULL SPECTRUM OF INTELLECTUAL TALENT IN THE COUNTRY. OUR TEAM INCLUDES INTERNATIONAL COLLABORATION AS WELL AS MEMBERS OF THE EXOPLANET AND HELIOPHYSICS COMMUNITY IN ORDER TO PLACE THE SCIENCE IN APPROPRIATE BROADER CONTEXT. URANUS WAS CHOSEN OVER NEPTUNE IN THE CURRENT V&V BECAUSE OF ISSUES INVOLVING TECHNOLOGY READINESS AND THE AVAILABILITY OF APPROPRIATE SPACECRAFT TRAJECTORIES THIS CHANGES IN THE NEXT DECADE AND HAS LED OPAG TO SUPPORT NEPTUNE OVER URANUS FOR A FLAGSHIP MISSION OWING TO TRITON A CAPTURED DWARF PLANET ITSELF AND ALSO A HIGH PRIORITY OCEAN WORLD TARGET . FOR THESE REASONS AND OTHERS EXPLORATION OF THE NEPTUNE-TRITON SYSTEM IS A PRIORITY FOR THE NEAR FUTURE. THE LAUNCH WINDOW TO NEPTUNE USING CHEMICAL PROPULSION ONLY IS OPTIMAL FROM 2029 TO 2030. FOR A NEPTUNE FLAGSHIP TO BE A VIABLE FIRST CHOICE FLAGSHIP MISSION IN THE NEXT DECADAL SURVEY AND TO MEET THIS LAUNCH WINDOW THE PROJECT MUST BE READY TO GO BEFORE THE INK IS DRY ON THE DECADAL SURVEY. THAT MEANS THAT THE SCIENCE OBJECTIVES MISSION ARCHITECTURE LAUNCH VEHICLE AND INTERNATIONAL PARTNERS SHOULD BE CLEAR. THIS STUDY WILL ILLUMINATE THE OPTIMAL PATH.