“2001” is episode ten of season five of Stargate SG-1.
In the episode “2010”, SG-1 changed history after an alien race, the Aschen, turned out to be not as friendly and helpful as they thought. The mission to the planet was scrubbed, and the gate address was locked in the database, but SG-1 has just run across them on another planet. The Aschen won’t disclose their planet’s location. If they did, SGC would know not to trust them. O’Neill doesn’t trust them anyway. Jack is not the most trusting of souls.
The SGC narrow down the Aschen’s homeworld as to being one of several – including the one locked from the database. Daniel and Teal’c discover the remains of a civilisation on the planet they met the Aschen on.
The SGC has to work out what the problem is, when they don’t know what happened and why.