To play Plumpin', simply move your mouse. Plumpin will be bounded to your cursor, and you move them to buttons to instantly press them, and avoid obstacles by dodging them with your mouse. Be quick, the level slowly scrolls away, and if your mouse goes off screen, Plump will freeze (yes you can move your mouse outside the screen and to another spot to teleport Plump but where's the fun in that?)
There are green glowing balls that'll give you shield mode for a few seconds (a progress bar on the top left will indicate how much shield mode you have). In shield mode, you are invulnerable to any damage, and you can attack the Red Quarter enemies (the red balls with eyes). Collide with the enemies with your mouse to damage them until they explode. Killing Red Quarters increases your shield mode time and heals you.