Number One is a hand-shedding game along the lines of UNO and Mau Mau. Each player starts with a hand of seven cards, with one card face up in the discard pile. On a turn, you must play a card that matches the color or number of the top card of the discard pile or else play a black action card; alternatively, you can choose to draw a card and if that card is playable, you may do so immediately.
Each color has action cards that reverse turn order, skip a player, or force the next player to draw two cards; in the latter case, if that next player also has a "draw two", then they can play it to force the player after them to draw four cards. Black cards can change the color of the top card, force players to pass all their cards in hand to the right, or do other things.
When you have only one card left in hand, you must say "One!" and if you don't and someone else catches you, you must draw two cards. Whoever plays all their cards first wins.