This is a story of fantasy whereby the characters visit other times and places through a portal in time. The main characters are George Pembroke, a retired well-to-do English gentleman from 1891; Helen Pembroke, a New York ex-stockbroker from 1929; and Bokatu, a flying saucer command pilot from the Pleiades Star Cluster (year unknown). Bokatu is introduced about halfway through the book as a main character. Till then, George and Helen Pembroke are the only main characters.

While on a stroll through the London fog, George discovers the time portal quite by accident. Even though the portal is quite evident to a select few, most people don't know that it exists. While George and Bokatu are quite at home in the portal and how to negotiate through it, Helen has to struggle to find her way around. George or Bokatu lead others through the portal and into different periods.

George and Helen often take their friends and club members to visit and explore other lands and other time periods. Most of the time, it's dangerous. Often, George and his friends are severely injured or killed in other times by the "local yokels" or by their constant nemesis, the Pleiades aliens. However, George and company find a way around this problem. And they don't hesitate to use this little trick of time.

George and Helen meet Bokatu in Roswell, New Mexico, in 1947, after Bokatu's saucer crashes. The three of them, in time, become the best of friends. George, Helen, Bokatu, and George's friends venture out on a time traveling trip every week. When they return, George's cook, Millie, prepares a meal for them. Then everyone retires to the parlor or living room for smokes, drinks, or conversations about time traveling.