No company approaches sustainability more comprehensively — or more creatively — than the British retailer Marks & Spencer.

M&S is the U.K.’s largest clothing retailer and a big seller of food, too (market share: 3.9 percent). It operates about 1,000 stores and employs about 78,000 people. Its supply chain includes 2,000 factories and 20,000 farms. Some 21 million customers visit the stores each week, and revenues last year were £9.7 billion ($15.7 billion).

The company’s sustainability effort, which is called Plan A — because there’s no plan B to protect the planet — touches executives, rank-and-file employees, customers and suppliers. Executive pay is based, in part, on meeting sustainability targets. Store managers compete to save energy and waste. Factories and farmers that sell to M&S are rewarded for going “green.” Increasingly, customers are invited to get involved, too.

“Plan A, at heart, is a change-management tool,” says Mike Barry, head of sustainable business for M&S.

I met Mike this week at M&S headquarters in London. M&S is making demonstrable short-term progress towards big long-term goals (about which, more below) but what stuck in my mind were these examples of how Plan A is changing the way the retailer does business:

Cleaning out the closet: M&S and Oxfam have teamed up to reward shoppers for recycling unwanted clothes bought at M&S. The clothes are donated to Oxfam, which raised about £3.3 million ($5.3 million) by reselling them. Anyone donating an item of M&S clothing to Oxfam gets a £5 voucher to use on a purchase of £35 or more on clothing, homeware or beauty products at M&S. This develops brand loyalty, and points to the circular economy of the future, where stuff is recycled and make into something else instead of being thrown away.

Selling energy, along with food and clothing: Capitalizing on the strength of its brand, as well as its reputation for sustainability, M&S has become a retailer of electricity and gas. “The first market proposition is great customer service,” Barry tells me, but once they sign up customers — they have “hundreds of thousands now” — they offer advice on saving energy and saving money, and then try to sell them energy-efficient appliances and home insulation. Again, customers get rewarded with M&S vouchers, so there’s a clear benefit to “green” behavior.

That carbon-neutral bra: M&S’s carbon-neutral undies, unveiled in April, are more than a marketing gimmick. They’re a way to showcase an energy-efficient  factory in Sri Lanka that was built as part of Plan A and powered, in part, by solar energy and hydropower. They were also a challenge for the company, as it delved into the complexities of carbon footprinting (Believe it or not, the bra contains 21 component parts from 12 different suppliers). To offset the CO2 generated by the bra’s manufacturing and shipping, M&S is planting 6,000 trees in Sri Lanka, some of which are lime and mango trees intended to generate income for farmers.