Bunny is your everyday pegasus who is clumsy and ditzy with poor grades in everything but flying. Her little brother, Kick Start is a lot smarter then her and can hardly fly. She meets a talking cat called Luna who gives her a magic necklace that turns her into Pony Moon, the champion of love and justice. She learns from Luna that she has to find her fellow four sailors and protect a mysterous Princess. And on the way she earns a little crush on a protector called Shining Mask and a enemy in a colt called Shining Protect.
Note:About why Usagi/Bunny doesn't have a cutie mark. It's because at this stage it be a dead giveaway and it wouldn't be a story then. Same with why she a pegasus not an alicorn.