Though it will support four-player cooperative (and even competitive) mystery-solving (think: Clue meets Zack & Wiki), the session I participated in was a two-player affair. From levels that include a train, mansion, aquarium, and other settings, I ended up playing the gumshoe character on a cruise ship that was missing its wheel and anchor. Along with my partner in crime ... solving, I came pretty close to solving the whodunnit (you need to find evidence as to the perp's hair length, sex, shape, and height, before accusing anyone) as I used my allotted moves and turns to investigate the dollhouse-like cut-away of the ship.
Special cards came into play (the over-arching story's villain might, for example, lock off a room -- there's a card that counters that) as did some ... err, unique "interrogation" mini-games, including "staring" into a suspects eyes, paying a bribe and tickling the truth out of an NPC using the Wiimote. The writing's sharp and funny, the classic adventure game underpinnings are noticeable, and I'm eager to play more.