What episode "Threshold"?
https://en.wikipedia.org/wiki/Threshold_(Star_Trek:_Voyager)
From Wikipedia:
"
Threshold" is the 31st episode...
...Voyager's crew discovers a rare, more stable form of
dilithium that they postulate could power a warp drive beyond
Warp 10. This would allow
Voyager to reach the
Alpha Quadrant near instantaneously. Although holodeck simulations prove disastrous,
Lieutenant Paris (
Robert Duncan McNeill) comes up with an idea after an off-the-cuff discussion with
Neelix (
Ethan Phillips). The next simulation is successful and a shuttlecraft, dubbed the
Cochrane, is prepared for a full test flight.
The Doctor(
Robert Picardo) identifies a rare medical condition in Lieutenant Paris indicating a 2% chance that he will suffer lethal effects from the test-flight and recommends assigning
Ensign Kim (
Garrett Wang) as test-pilot. Paris convinces
Captain Janeway (
Kate Mulgrew) to allow him to fly the shuttle despite the small risk.
Paris successfully breaks the Warp 10 barrier with the
Cochrane, rapidly disappearing from
Voyager's sensors. The crew begins to try to track the shuttle, but soon the
Cochrane reappears, Paris unconscious at the controls. Once awake, Paris explains that he had seen everything at every point in space, and the shuttle's database similarly contains a massive amount of information about the
Delta Quadrant. However, Paris starts to suffer allergic reactions, and he is raced to
Sickbay, where the Doctor determines that Paris is now allergic to common water. Paris's body soon changes again, and no longer can process oxygen, forcing the Doctor to create a special environment that Paris can exist in.
Paris's body continues its strange transformations, the Doctor postulating that he is becoming a new form of life. Before the Doctor can use an "anti-proton" treatment to return Paris to his human form, Paris escapes, disrupts
Voyager's internal systems, and kidnaps Janeway on the
Cochrane. By the time the crew restore the damage Paris had done, the
Cochrane has taken off to Warp 10. As
Voyager follows the shuttle's trail, eventually coming to a planet covered with swamps, the Doctor explains that the mutation patterns in Paris' DNA are consistent with those of evolution. Near the shuttle, they discover two amphibian beings, with trace DNA of Paris and Janeway. The two have mated and have had three offspring. The crew recover their transformed crew-members to be returned to human by the Doctor, and leave the offspring behind.
Congrats though 
