Flatland; a romance of many dimensions is a short novel by Edwin Abbott published in London in 1884. Abbott described his interactions with beings who live in a flat plane of two spatial dimensions, and showed how we might perceive the existence of higher dimensions.

Abbott wrote Flatland before the unified picture of space and time emerged in early 20th century mathematics and physics. Nowadays we would say that the Flatlanders live in 2+1 dimensional spacetime -- two space dimensions and one time.

Einstein's General Theory of Relativity is much simpler in Flatland (plus time), so that is where we'll start exploring what it means to travel in time.