The start player plays any card to the center of the table. The next player must play a card at most ten lower or higher than this card, e.g., playing 14-34 if the 24 was lead. If the second-played card is lower (higher), then all subsequent cards must be lower (higher) — but also always within ten of the most recently played card. If a player cannot or doesn't want to play, they pass and draw the top card of the deck, then reveal the new top card. If all players pass in turn, then whoever played the most recent card clears the table and leads a new card.
Two exceptions:
1. If the next player in turn can play a green card exactly ten lower or higher than the topmost card on the pile (no matter the direction of play), that player can call "Mount Ten!", play the card, then clear the table and lead a new card.
2. If any player can play a red card exactly ten lower or higher than the topmost card on the pile (no matter the direction of play), that player can call "Mount Ten!", play the card, then clear the table and lead a new card.
When one player has no cards in hand, the round ends. Each player with cards in hand scores -1 for each green card and -2 for each red card; the player who went out scores positive points equal to all the negative points scored that round. Complete as many rounds as the number of players, with the start player rotating each round. Whoever then has the highest score wins!
You're also watching the cards fly by as the design functions something like a real-time game with players trying to get something onto the table before someone else "MountTen!"s them out of turn — which in a five-player game happens a lot since forty cards start in play. (Which is why you have to say "MountTen!" in the first place as otherwise the game turns into Dutch Blitz, with people slapping out cards to beat others to the punch, bending your cards, then looking sheepish because they forgot the rule about saying "MountTen!") With fewer players, it's easier to track who's played what and make plays accordingly
Naturally you feel compelled to hold onto the red cards over the green since you can possibly play them out of turn — thereby stealing the play and getting rid of two cards in one go — but that's why the penalty for holding red cards is higher. Good and bad are merged in one element, adding a bit of tension to what you're doing on every play, which is what you want in a design. That said, you don't need to hold red cards to win a round, so you have to learn to let go based on what's actually being played.
One oddness with the cute artwork is that the cards are indexed on only a single corner, with the opposite corner having a giant number that you can see only by fanning your cards out across both hands.
The publisher labels MountTen as being playable by ages six and up, but either Japanese children have giant hands or their parents smartly bring card holders to the table to make things easier for them. Me, I didn't do that, which made things tough for my 6yo. Lesson for the future...