Poor households do not chiefly need small sums, Rutherford argues; they need occasional large ones. Life-cycle obligations, emergencies from illness and bereavement to fire, flood or the bulldozing of a slum, and opportunities ranging from a business to a better roof or a bribe for a secure job all demand more cash than a home can hold.

Since the poor own little to sell or pawn, the dependable route to a lump sum is to build it out of savings. Rutherford calls this basic personal financial intermediation and reduces it to three moves: saving up, borrowing against savings not yet made, and combinations of the two. The rest of the book follows from that.

He then surveys what people have devised without banks, from rotating and accumulating savings clubs to deposit collectors paid handsomely to take money away and keep it safe, and describes SafeSave, designed around tiny, irregular, daily transactions.