In case you skip it...

As implied by the ending of “Tomorrow, and Tomorrow, and Tomorrow”, Past Kelly’s memory wipe didn’t take. Since she turns down Ed’s advances, the timeline changes - they don’t marry and divorce and Ed doesn’t end up in command of the Orville. This results in Claire Finn not serving about the Orville and meeting Isaac, so he never forms a relationship with her and her children. Thus he does not betray the Kaylon when they attack and they successfully eradicate all life on Earth.

This episode picks up nine months after that with the Planetary Union all but destroyed. The survivors include the crew members we know from the rest of the series, but they are scattered and spend most of their time fleeing Kaylon forces.

Kelly still remembers how things were supposed to go in the original timeline and pieces together what went wrong. She rounds up the people she knows as the Orville’s crew with a plan to go back in time and wipe her memory correctly to restore the original timeline.

It works. They fix the timeline.

Unfortunately, the episode is not very compelling. Most characters are very similar to who they were in the original timeline and the few differences are unsurprising, so we don’t really learn anything new or interesting about them. There are no moral dilemmas or social allegories to explore. The on-screen events are theoretically dramatic, but it’s hard to care much about them because there are no real stakes. Obviously the show isn’t going to stay in the bad timeline so of course they are going to succeed, which means nothing in this episode actually matters.

It’s easier to just pretend that the memory wipe at the end of the previous episode actually worked - you end up in the exact same place and miss out on very little.