The Gary Seven incident, Picard's temporal double, O'Brien's flash-forwards into the future and the 29th century Timefleet all serve as examples for running time travel plots in the Star Trek style.

The book also features some thoughtful extrapolation of the Star Trek background. The "Department of Temporal Investigations" seen in the DS9 episode "Trials and Tribble-ations" is fleshed out in detail, providing enough rules and background information for games entirely based on the adventures of time-hopping Federation agents.

There are also brief presentations of several alternate timelines, from relatively familiar worlds such as a universe where Edith Keeler survived to worlds where the Vulcans were conquered by the Romulans or Khan became ruler of the Terran Empire.

Time and the final frontier

Despite suggesting some strange twists on the history of the Federation, everything about All Our Yesterdays feels true to the universe, from the discussions of temporal physics to the adventure plot seeds scattered throughout the book.

Perhaps that's because time travel is such an fundamental element of Star Trek. The series may herald space as the final frontier, but many of its most compelling stories have been about time.

The best Star Trek stories have always been about choices, and these are at the heart of most time travel stories.

Time travel stories balance a choice of futures. They make the movement of history personal by highlighting scenarios where a single person or action can change the world.

In "The City at the Edge of Forever", Kirk is forced to decide whether Edith Keeler will live or die. The story could have been told in the "present" of Star Trek -- Keeler could be a social activist on an alien world -- but the consequences of her death would be couched in terms of "maybe" and "perhaps."

The magical device of time travel raises the dramatic stakes. By putting the decision in Kirk's "past," the historical results of his decision are clear to him.

The effect is best summed up by the episode itself. As Edith Keeler dies, McCoy angrily asks Kirk, "Do you know what you've done?"

"He knows, Doctor," is Spock's reply. "He knows."

A choice of futures

In a well-run roleplaying game, of course, players make choices all the time. Star Trek games have a fairly rigid style, however, and roleplayers sometimes see their actions and the consequences that follow as inevitable.

A time travel scenario can shake up that sense of inevitability. The plot devices suggested in All Our Yesterdays highlight the possibilities of a player's decision just as well as they do for a character in one of the Star Trek series.

Time travel stories have their pitfalls, and they can certainly be overused. But occasional time travel stories can provide a quintessential Star Trek experience, and All Our Yesterdays is a valuable guide to what makes them work.