Plus is a classical number game also known as Twenty-One. Fill cells with numbers and get 21 in each of them. If you get 21, score from the cell moves to game score. Cell can be used again and again. Any number bigger than 21 blocks the cell.
Plus allows you to use two powerups. Turtle slows down game pace for several seconds. Use it if you can't aim well because numbers fall too fast.
Second powerup clears the cell. Tap one of your cells when it turns red and it will become available again.
Don't forget that each powerup costs some credits. You can earn credits by getting good results in previous games. Also, you can buy them or get for free by watching short ads.
Mobile version allows to use gravity. It's turned off by default, but you can turn it on in game settings (Options screen).
Gravity detects orientation of your phone. Numbers fall faster if you hold it vertically. You can change falling speed dynamically by tilting your phone.
Gameplay changes during your playing. Numbers start to fall faster, but you also get more score for number 21.
When some of your cells are blocked, you have less available options to drop flying number. But most of numbers also become smaller.
Compete with your friends. Beat their score and become the best Plus player.
Challenge yourself and get all the achievements. To get an achievement, fullfill it's requirements. Clear 3 cells in a row, reach full speed or beat some score.