Your little ritual at the end of the day. Solve tonight’s Sudoku and earn a star in your sky. Keep coming back and your stars slowly draw constellations. A cat keeps you company from the observatory window while rain taps the glass and lo-fi plays on the radio. And when one puzzle isn’t enough, 5,000 more are waiting for you.

Every night, everyone gets the same Nightly Sudoku. Each constellation takes 7–12 nights to complete, there are four to choose from each month, and every star you earn stays yours.

Miss a night? Your constellation simply waits for you. A puzzle finished after midnight still counts for the night you started it, and a star earned under a full moon shines a little brighter.

5,000 more Sudoku puzzles come with the game:

• 1,500 Easy
  
• 1,500 Medium
  
• 1,250 Hard
  
• 750 Expert

No repeats until you’ve exhausted a difficulty. Every puzzle has one solution, reached through logic alone. No guessing required. Easy and Medium ease you in, Hard asks more of you, and Expert introduces techniques such as pointing pairs and subsets.

There are no pencil marks. Every number is checked as you place it: correct answers lock in, while mistakes clear themselves. On the Nightly, each mistake adds 30 seconds to your time; on endless boards, the clock never affects your score. Your first mistake is explained, and each board gives you one to three hints, depending on difficulty. Take your time.

Between puzzles, it sits on the observatory’s window ledge. While you play, it wanders along the top of your Sudoku board, watching every move. Leave it alone long enough and it falls asleep right where it sits.

Click to pet it. Right-click, or click and hold, to change its coat: there are eight, from a plush gray Chartreux to a longhaired Ragdoll. Make a mistake and watch it wince. Get twelve correct answers in a row and it gives you a slow blink.

Some nights you’re solving Sudoku. Some nights you’re keeping the cat company.

Ten worlds wait beyond the observatory window. Watch cherry petals drift past the glass, settle in with rain falling outside, let fireflies glow in the dark, or look up for shooting stars. Change the view with the brass arrows beside the porthole, or tap the bracket keys while you play. Your Sudoku stays the same. The night changes around you.

MoonDoku FM has three stations: Chillhop, Lo-fi Jazz and Downtempo. Add the sounds that suit the night (rain, a cozy café, a crackling fireplace, ocean waves or a distant storm), each at its own volume, so the room is as quiet or as atmospheric as you like. It all works offline.

When you want nothing but the puzzle, press F for Focus. The room fades away while your music keeps playing. Just you and the Sudoku.

• 5,000 Sudoku puzzles are yours to keep. They live on your drive and work offline, with no account required. The Nightly is the one exception: it comes from the server, so it needs an internet connection.
  
• Steam Cloud keeps your sky, your streak, your cat’s coat and your half-finished puzzle with you between machines.
  
• Keyboard, mouse or controller. Type your numbers, click the 3×3 pad beside the board, or use a controller’s 3×3 picker.
  
• 17 achievements to collect, from your first Easy solve to a thirty-night streak.
  
• An optional free account. Want to see how your Nightly time compares? Sign in with an emailed code to see where you rank after each solve.
  
• One purchase. No ads, no in-game store, nothing sold separately. No subscriptions, no energy system, no interruptions.
  
• Windows, macOS and Linux.

Put the kettle on. Your cat is waiting by the window, and tonight’s puzzle is ready.