An unidentified object closely follows Voyager. Seven of Nine identifies the phenomenon as a huge mass of subspace energy that is attracted to objects that emit electromagnetic energy. In order to avert the graviton ellipse, which travels through subspace, Voyager must cut power and reverse their shield polarity.
After a successful aversion, Chakotay and Paris remind Captain Janeway of a similar phenomenon back in 2032, in which a command module from one of the early Mars missions was consumed by a rolling ball of deadly energy.
Captain Janeway decides to launch a search for the missing U.S. spacecraft trapped inside the energy field. She instructs Seven to join both Chakotay and Paris, as they hunt for the legendary Ares IV, the command module from the first manned mission to Mars. Janeway hopes that Seven will develop an appreciation for studying the past.
Before they leave on the Delta Flyer, Chakotay and Paris view NASA's footage, which recorded the last seconds of Lieutenant John Kelly's life within Ares IV. The Delta Flyer finally enters the mass of energy, but not before encountering excessive turbulence. Once inside, asteroid fragments and pieces of captured vessels come into its view.
The exploratory crew has approximately five hours before the energy ball heads to another subspace. However, back on Voyager, Torres discovers that a dark matter asteroid is heading straight for the mass of energy. With only minutes to spare, Captain Janeway immediately instructs the Delta Flyer to break out. Chakotay goes against the Captain's orders and decides to try to bring the module back with them. Unfortunately, the weight of the Ares IV inhibits their swift escape, and the Delta Flyer is unable to make it out in time without being affected by the meteroid's impact.
Chakotay has been knocked unconscious and the Delta Flyer is badly damaged. With their engines down and their shields off-line, Chakotay, Seven, and Paris now only have two hours before the mass of energy returns to subspace.
Finally, Voyager is able to re-establish contact with the Delta Flyer. Seven is clearly dissatisfied with Chakotay and his decision to disobey the Captain's orders. After discussion with the Voyager crew, their only hope is to beam Seven over to the Ares IV to obtain its ion distributor in order to supply power for the Delta Flyer. Before Seven is beamed over to Ares IV, Chakotay asks her to download whatever she can from the module's database.
Once inside the Ares IV, Seven enters the cockpit where she sees the body of John Kelly still strapped in his chair. Seven brings the main computer online, and a monitor immediately comes on which contains an active datafile. Chakotay asks Seven if she could playback the video log entries for them.
The video entries reveal that John Kelly was not killed on impact with the mass of energy. Instead, he lived for many more days until his life support system ran out. Due to power failure, the module was unable to escape. John Kelly saw other spacecraft within the mass and discovered before his death that the human race was not alone.
Seven is successfully able to obtain the distributor. She also decides that she will beam John Kelly's body with her back to the Delta Flyer. With only minutes to spare, the power conversion is activated. Voyager gets as close as it can to the mass to help rescue the Delta Flyer as the ball begins to retreat into subspace.
Luckily, the Delta Flyer makes it out just in time. Back on Voyager, Captain Janeway and the rest of the crew pay their respects to Lt. John Kelly and they release his body into space. Seven is able to acknowledge that John Kelly's contribution helped secure humanity's future.