Theory of Mind (ToM) is crucial for understanding social dialogue, where characters’ mental states are rarely stated explicitly but evolve through interaction and shape later behavior. Existing ToM benchmarks focus mainly on selected dialogue turns or state changes across separate scenarios, leaving within-dialogue mental state trajectories underexplored. We introduce TrajToM, a benchmark for evaluating whether LLMs can reason over evolving emotions, beliefs, intentions, and desires in continuous social dialogue. Built with a Sketch-first construction paradigm, TrajToM contains 700 dialogues and 18,165 multiple-choice questions across mental state reasoning, behavioral reasoning, and trajectory reasoning. Experiments reveal a substantial human-model gap, with GPT-5.2 reaching 67.4% overall accuracy compared with 88.2% for humans, and show persistent bottlenecks in second-order reasoning, behavior explanation, and change direction assessment. We further examine MARTA as an exploratory trajectory-aligned process-supervision framework, showing that explicit supervision over mental state trajectories can improve ToM reasoning.