A mad scientist named Dr. Killbait succeeded in resurrecting one of the eight ancient dragons, endangering the country of Hyneld. The captain of the king's knights, Robin Solford, witnesses the dragon's attack on the castle. His two comrades are killed, and the princess Patricia, Robin's adoptive sister, is nowhere to be seen. Robin is determined to find Patricia and stop Dr. Killbait, and begins to wander around the world, hoping to find friends who will help him on his perilous journey.

Startling Odyssey II: Mary? Sens? is not a sequel, but a prequel to the first game, Robin Solford being the father of the previous game's hero. The game is set in a predominantly medieval fantasy-themed world with a touch of sci-fi, and plays like a traditional Japanese RPG. The player navigates a party of characters on a top-down overworld, visiting cities and dungeon-like areas. Enemies are encountered randomly. Battles are viewed from isometric perspective, with an "auto" option to speed up combat.

Characters join the party as dictated by the plot; the party will eventually contain four active combatants. Each character has his or her own sets of offensive, supporting, or healing spells, which are learned automatically when the character levels up. The game features full-screen anime-style cutscenes and voice acting during important events.