Taken from Wikipedia.org:

As the game begins, the player's character is spending a final day of a London vacation in the Kensington Gardens. The evening flight back to the United States is looking increasingly unlikely for a number of unusual reasons. Hordes of nannies are blocking all exits from the Gardens, and the grass actively resists efforts to be walked upon. Worst of all, a gleam on the horizon soon heralds the unwelcome arrival of a Soviet nuclear missile. Time begins to slow as the missile approaches, and with some ingenuity the player's character finds an incongruous door hovering in mid-air. There's no telling where it may lead, but it cannot possibly be worse than the alternative of being at ground zero of a nuclear detonation…

The doorway leads to a strange land, where impossible objects exist. Space and time do not seem to behave in the familiar ways here. Exploring this new environment, the player finds several other mysterious doors, each of which leads to another chapter in the history of nuclear weaponry. After visiting test sites (including ones in Siberia, Nevada, and the Eniwetok Atoll) and Nagasaki just before each device is detonated, the player has one scenario left to deal with. The final door leads to the New Mexico desert on July 16, 1945, mere minutes before the test-firing that will change the course of history. But something is wrong at the “Trinity” site, and without the player's intervention things will go horribly awry.

Trinity_(video_game)#Plot