- The war setting for the flashbacks. Angel was never previously involved in a war, so they had to shoehorn him into WW2 by getting contacted by The Initiative and drafted. Mal, however, is well established to have a war in his backstory.
- The submarine setting for the flashbacks. The process of getting Angel onto the submarine is somewhat nonsensical - sure, vampires don't need to breathe and would be at least heavily resistant to the cold of the depths, but the pressure would still be an issue. And they knew exactly where to drop him to get him onto the submarine? And he could somehow get in through the torpedo tubes? It's a big stretch. If you place it on a spaceship, though, Mal already has good reason to be there.
- For that matter, the present-day portions make more sense if they're on a spaceship too. While Wolfram and Hart security is well-established at this point in the series to be swiss cheese, the fact that no one interrupted Lawson while he was capturing and stringing up Gunn, Fred, and Wesley strains credulity. If the cast is isolated on a space ship, again, this works a lot better.
- The theme of duty and purpose. Common in Firefly, less so in Angel.
If you change the vampires to reavers, the same basic plot still works. In the flashbacks, a ship containing Mal and Lawson (and probably Zoe) has been invaded by reavers, many of the crew are dead, but a few are still holding out and trying to survive. Instead of siring Lawson to save the ship, Mal either left him to die to reavers in order to help the rest of his crew escape, or worse, ordered him into the reaver-infested portion of the ship to perform some critical duty to save the rest of the ship. Mal and the rest of the remaining crew survive and escape the reavers, but Lawson is left behind with the Reavers and assumed dead. Then in the present day, Lawson, now a reaver or very nearly (like the reaver survivor from "Bushwacked"), invades Serenity, captures a few of the crew, confronts Mal, and Mal ends up killing him. Mal and another crew member (probably Zoe, Inara, or Book) then end the episode by having essentially the same talk about duty and purpose that Angel and Spike do.
