In March, 1845, Thoreau decides to build a cabin by Walden Pond, near Concord, Massachusetts, thus beginning his so-called "personal experiment." His goal is to discover everything he can about human nature; he thinks he can do this best when he doesn't have to deal with normal worldly concerns, like material goods and human society. Basically, he's super-crunchy.
The book is largely structured around the seasonal changes that Thoreau observes during his two years at Walden Pond. His days are filled with things most of us would never take the time (or have the opportunity) to do, like farming and observing different flora and fauna – that basically means plants and animals – that inhabit the area. Sometimes he puts his party hat on and goes into the village. On one such occasion, he's actually arrested – oops – and spends the night in jail for not paying a poll tax. (Please hold while Shmoop checks what a poll tax is and then makes sure we've paid ours...)
When he's not contemplating life, Thoreau sometimes entertains friends at his cabin. Throughout the book he meets quite a motley crew, including a poet, a philosopher, and various settlers, hunters, farmers, and laborers who offer him stories about the area's history.
But back to nature, where his loyalties lie: Thoreau takes the time to explore numerous ponds in the area, including Flint's Pond and White Pond. He also checks out the local farms, like Baker Farm, where he briefly takes shelter with an Irish laborer and his family. By the fall, he notes how the colors of the trees have changed, and he prepares for the winter by finishing the chimney on his cabin. During the winter, he observes Walden Pond in its frozen state, and is careful to notice the changes occurring around him. When spring finally arrives, Thoreau writes about how the frozen earth melts right before his eyes. There are also a ton of other changes that come with spring. For example, more varieties of birds and animals are present, and pine trees begin to pollinate.
Not much changes the second year he's there (although we don't get as much juicy detail about it). And alas, Thoreau bids a fond farewell to Walden in September, 1847.