The Enterprise makes way toward a planet known as Penthara IV, where a massive asteroid has struck an unpopulated continent. Fearing that the resulting dust cloud could lead to a devastating nuclear winter like the one that occurred on Earth in the late 21st Century, the crew hopes to find a way to counteract the cloud's effects. As they travel toward the planet, a bright flash appears on the Bridge, and a strange man materializes before the crew. The man reveals that he is a Professor Berlinghoff Rasmussen, an historian from the late 26th Century Earth, who has traveled from the future to study the Enterprise. Although the crew is suspicious of Rasmussen, his presence arouses their interest in the future, but Picard insists they refrain from questioning their guest so they don't end up changing the future.
The Enterprise arrives at Penthara IV and the widespread devastation caused by the asteroid hit becomes apparent. While Geordi La Forge and Data work to save the planet, Rasmussen asks the crew to complete lengthy questionnaires, ostensibly for his research. He also asks to see several pieces of equipment, regarding them as relics of a bygone era. The crew members are nothing but polite, but privately they begin to lose patience with Rasmussen and his methods. Troi is especially wary of the man and what his true intentions might be after picking up a vibration that he is trying to confuse them.
When the crew's initial plan to save Penthara IV backfires, Geordi comes up with an alternative solution that will either completely save the planet or kill every living thing on it. Desperate, Picard asks Rasmussen for advice regarding the plan, but Rasmussen refuses to help him on the grounds that Picard is trying to manipulate the future. Left only with his original choice to act or not to act, Picard goes with his original impulse and orders the crew to implement the risky plan.
The crew's efforts are successful, and Penthara IV is saved. Rasmussen immediately prepares to leave the Enterprise, but Picard and the crew greet him outside his vessel with the announcement that they plan to search his ship for several objects that have mysteriously disappeared during Rasmussen's stay. Rasmussen relents, but insists that only Data be allowed on his vessel since, if so instructed by Picard, he will never divulge what he sees.
Once inside, Rasmussen pulls a phaser on Data, revealing that he is not a 26th-century historian but a 22nd-century inventor. He traveled to the Enterprise to steal technology that he will take credit for creating, and he now plans to take Data with him as well. Luckily, Picard has already deactivated the phaser, and Data is able to outsmart his foe. After disposing of Rasmussen's ship, the crew turns him over to the authorities.