Time is truly of the essence when the U.S.S. Enterprise-D is visited by Professor Berlinhoff Rasmussen, a time traveler. The Enterprise detects Rasmussen's time pod as the ship is on a rescue mission to Penthra 4, where an asteroid struck and created a devastating "nuclear winter" effect. Rasmussen claims to be a historian from late 26th-century Earth, and exhibits a familiarity with the smallest details of the Enterprise and its crew. Despite the crew's intense curiosity, he does not reveal future events or the reason for his visit, saying he would risk altering his own past. Couselor Troi senses Rasmussen is hiding something more than just his knowledge of their future, but cannot figure out what it is.
Rasmussen makes a pest of himslf by observing and questioning everything the crew is doing during the Penthara mission. He even gives "homework assignments" to the senior staff, covering basic questions about technology to their reflections on scientific advances since the 22nd century. At times, he surreptitiously steals small items such as a data PADD and a tricorder.
As the crisis on Penthara worsens, Picard's only means to save the planet's 20 million inhabitants is a procedure that runs the risk of burning off the planet's atmosphere. Picard asks the professor for advice, since Rasmussen knows the outcome. The professor regretfully is unable to help. Saying Rasmussen has shown him the value of the right to choose, Picard chooses to attempt the procedure, which succeeds and saves the planet.
Picard and his staff, their suspicions aroused by the many missing items, confront the professor and demand to search the time pod. Data accompanies the professor inside, where Rasmussen draws a phaser and reveals he is not from the 26th century. He is, in fact, a "dismally unsuccessful inventor" from the 22nd century, who stole the time pod from the real 26th century time traveler and came forward in time to steal technology for himself. He planned to become wealthy by "inventing" all of the stolen items himself. Data easily captures Rasmussen, and the time pod returns to the 22nd century without the inventor. As Rasmussen is led to the brig, Picard welcomes him to the 24th century, his new home.
