Aphelion is a text-forward space-horror adventure in which Vale, a rescue medic, boards the survey ship Aphelion-7 ten weeks after it went dark past the heliopause. Told in first-person present tense with painterly scene art and visual-novel dialogue, the game moves compartment by compartment through a dying ship, gathering evidence and reading survivors who may no longer be entirely themselves.

Around 21 pen-and-paper puzzles with provable solutions gate progress, spanning logic-gate circuits, ciphers, and constraint locks, and hints cost sanity, a resource that makes the game itself start lying as it drops. Choices persist across the story, two of them irrevocably, leading to six tonally distinct endings tracked across playthroughs.