It is Christmas time, and the rich, pampered, adult March girls-- Meg, Jo, Beth, and Amy-- are unhappy because they have been pressured into going to work. They have done this because their father has lost his fortune, and they have no means of support and very little money.

On Christmas morning, the girls work as ponygirls and milkmaids to some needy neighbors, the Hummels, plowing their fields and supplying them with breast milk. This act of kindness is noticed by their wealthy neighbor, James Laurence, who sends them expensive lingerie and horse tack. That afternoon, the girls learn they are to wear their new lingerie and tack and perform as showponies for the village men in the town meeting hall. As reluctant as they are beforehand, they are pleased afterwards that they were such a hit, bringing in much-needed funds.

The March girls are then invited to a party at the Laurence house. They dress up in their best and attend. James Laurence's grandson, James Laurence III, coerces the four girls into becoming cart ponies for his carriage for their journey home, reminding them that, if it wasn't for the gifts of lingerie and tack, they would not have been able to earn all that money for their father. On the way, Meg sprains her ankle and is terrified she will be put down, but is able to limp home with the others.

After the holidays, the girls go back to their various duties, grateful that their ponygirl service is behind them: Meg is a governess, Jo is a companion for Aunt March, Beth studies at home and helps with the housekeeping, and Amy goes to school.

The Laurences decide one day that the pretty young March girls are missing their true calling, and that-- based on their charitable service on Christmas day-- they should become permanent ponygirls. They lure the girls to their house, where they have the servants strip them naked, then harness and bridle them.

The frightened March girls realize they had been tricked as they are held prisoner in the Laurence stable and trained as ponygirls. The sisters expect their father to save them, not realizing he has been paid a handsome sum as compensation. When no help comes, they plot to escape, but are unable to, and are soon loaded into a horse trailer and shipped off to be sold.

Meg, the oldest, is very pretty and interested in the finer things in life. She is given a massive boob job by her new Master and has her head shaved and braided into a stylish mane. Stripped naked, she is held in place by nipple and labia rings that are tied to a metal ring in the floor. She weeps at the loss of her beautiful clothes and other luxuries.

Eventually, she is groomed to become a Breeder. Her head is shaved completely, and she is kept prisoner in a tiny stall, where she is forced to endure hours of constant sexual stimulation, her helpless body at the mercy of giant realistic vibrating penises that are painfully large. She knows what they are preparing her for, and cannot bear the fact that these sex toys will soon be replaced by non-stop sex with anonymous ponyboys.

Jo, the second oldest, does not take to life as a ponygirl, even though she is the tomboy among the four sisters, as she enjoys her freedom and hates being told what to do. She hates the tight leather restraints and often rebels, actions for which she is often whipped and denied privileges.

She and her sister Beth-- who is very shy and timid-- are forced to become naked cart ponies. They are furious that such privileged girls as themselves are forced to work like beasts of burden, carrying riders around their Master's estate while naked and bound, and they abhor their buttplug ponytails and their tiny labia bells, but there is nothing they can do.

Their Master decides that their tits are too small, and gives them both huge implants. Beth, in particular, is shamed, because it draws attention to her and her body, causing great distress for the shy, mousy girl. She doesn't like being the center of attention, but trotting around the estate with her massive mammaries bouncing like basketballs prompts lewd comments from everyone who sees her.

The rebellious tomboy, Jo, is made to sexually please her Master in a final effort to force her into submission. She finds this extremely distasteful, but knows she has no choice but to comply. Her spirit finally broken, she does what she can to make the best of the situation, allowing him to violate her in every way possible.

Jo eventually learns to accept her situation, and becomes an excellent cart pony. Beth, unfortunately, has been unable to adjust and takes a turn for the worst, and has to be put down. Jo buries her grief by becoming extremely obedient. She knows, too, that she is expendable, and could suffer the same fate as her sister should she fail to please Master in any way.

Amy, the youngest at 18 years of age, had visions of becoming an important artist. Her ego suffers a severe blow when she becomes just another ponygirl on a remote farm with dozens of others. Her state if mind is further damaged when she is branded and given a ponytail, humiliating her.

But it is Amy's luck to be chosen as a showpony. Her head is shaved, and her tits are surgically enhanced until each one is the size of her head. She becomes a celebrity, and is trotted out for many important events. Though she is unable to exercise her creativity, she soon learns to accept her new-found fame.

A year later, the three remaining sisters are re-united at a family reunion on the March estate, which the father had been able to keep based on the money paid to him by the Laurences' for his girls. Viewing his daughters in all their naked and obedient glory, he concludes that things have worked out for the best, and gives them each a sugar cube as a reward.