The ISS Enterprise (NCC-1701-A) was a Constitution-class starship commanded by Commander Pavel Andreievich Chekov in the late 23rd century. On an unknown stardate, the Enterprise encountered the ISS Excelsior (NCC-2000) inside the Janus Prime Vortex. Shortly after the Excelsior's arrival, the Enterprise destroyed the ISS Ranger.
After Excelsior's fighters destroyed the rest of Enterprise's fighters, the Excelsior along with her fighters exited the vortex, which subsequently collapsed.
Chekov's bad luckEdit
Chekov was actually chasing the Captain Sulu from the "primary" (Federation) universe. An event within the Janus Vortex in the primary universe (a dilithium crystal shattering aboard Excelsior) caused the crews of the USS and ISS Excelsior to switch places. The only way to reverse this was the Janus Ultima Vortex in Tholian space.
No matter how hard Chekov tried, that one fighter from Excelsior kept causing him to have to retreat and not be able to destroy Excelsior. The fighter ended up picking off all his fighters. However, during a sunstorm in Romulan space, Chekov found out about the switch, but because he did not destroy Excelsior (again), Chekov was ordered to break off his pursuit of Excelsior and head to Tholian space, where he was trapped in a Tholian web.
Excelsior saved the Enterprise from the web despite the fact the Enterprise kept trying to destroy her. Chekov explained to Sulu what he had found out and was willing to help Sulu get home. Before Sulu's return home, the Excelsior and Enterprise duked it out with the Empire one last time.
All of this takes place in the video game Star Trek: Shattered Universe.