Following a series of holodeck simulations to reach Warp 10, Paris and the engineering team iron out the technical glitches and prepare to send Paris out on a real flight. The next day, the crew watches as Paris takes the Shuttlecraft Cochrane to Warp 10, crosses the transwarp threshold, and abruptly vanishes.
Moments later, the Cochrane emerges from subspace, and they beam Paris to Sickbay. He appears no worse for wear, weakened but exhilarated by the experience, which he likens to "being everywhere at once." As Torres and Janeway discuss the potential of the data obtained in Paris' brief flight, a crewmember named Jonas eavesdrops on the conversation. Later, the duplicitous Jonas sends information on Paris' Warp 10 flight to the Kazon.
Not long after his trip, Paris collapses in the mess hall. Rushed to Sickbay, the Doctor tracks dramatic changes in Paris' biochemistry. His organs are mutating and his cell membranes are deteriorating rapidly. Despite the Doctor's best efforts, Paris dies. However, hours later, Paris begins breathing again. When the Doctor examines him, he is amazed to find that the Lieutenant now has two hearts!
Paris is by no means out of danger. A series of accelerated mutations leave him radically transformed and subject to bouts of paranoia and violence. The Doctor figures out a medical procedure to destroy Paris' mutant DNA, but it's interrupted when Paris breaks out of confinement.
Paris kidnaps Janeway and takes her to the Cochrane, launching them both on another Warp 10 journey. Voyager locates the vessel three days later on an uninhabited jungle planet. Paris and Janeway have mutated into amphibians and mated, producing two offspring, which the crew leaves behind on the planet. Once Paris and Janeway are brought back to Voyager, the Doctor is able to perform the procedure to eliminate the mutant DNA from their bodies and they return to normal, albeit rather embarrassed about recent events.