I thought this one was really good, and not just because Captain Mercer comes from Massachusetts (although that helped). The time travel plot was interesting for several reasons. First, the antagonist was not out to hurt anyone and not even out to steal anything-- she specifically chose vessels that would be destroyed and saved the lives of all aboard. In contrast, by destroying the singularity, Mercer killed countless people, possibly including the time traveler. Another interesting aspect is that the Orville and crew survive now in an altered timeline-- they're literally living on borrowed time.
Other things I loved about the episode were the comet rescue at the beginning (laws of physics aside), which was one of the most exciting space sequences I've ever seen and beautifully rendered. Also, the dark matter sequence was beautiful. The simulated alien environment was gorgeous. The singularity was okay, but the alien ship and the aliens on the other side were terrific. Most of the time, advanced CGI effects are pretty boring because the technicians go for photorealism and the results are impressive but lack artistry-- but the effects on this show, especially this episode, are very creative.
The episode also included a classic TOS "He's dead" fade out.
Some of the humor did fall flat, as usual, but not as bad as in some other episodes. The subplot about teaching Isaac practical jokes was actually pretty good. The Mister Potato Head scene was funny, but the leg amputation, especially Malloy's rampage on the bridge, was horrific-- only to have him reconsider and tell Isaac's lifeless body that it was the best practical joke he'd ever seen. The part where the leg fell from the ceiling was a bit too much, though. Probably the part where Malloy came to the bridge with a limp, half-formed leg was probably too much, too, but it cracked me up.
And mixed in with all that was a nice little story about Mercer-- who we find out has not been with anyone since his divorce-- gets burned when he tries to move on, and has to make the hard choice at the end.