The true nature of Norm's marriage to Vera is one of the truly inexplicable aspects of the series. By the final seasons, Norm was unsure of Vera's maiden name and whether his father-in-law was still alive. This episode is probably the last time Norm and Vera's relationship was taken seriously.
Norm and his neighbour Phyllis are starting to suspect that their respective spouses are having an affair. They eventually decide to hire a private eye to check up on Vera and Ron, and Norm spends most of the episode stewing over whether Vera is actually unfaithful. It's not exactly a hilarious premise, but there are a few good lines.
The subplot isn't much but it's pretty funny. Sam does a radio interview with his old buddy Dave, and refers to Diane as a former "Love Bunny," which naturally displeases Diane. Sam's awkward apology ("I apologize to her and indeed to women everywhere, both living and dead, whose sensibilities may have been offended") is yet another sign that, despite their protestations, Sam and Diane are destined to be together (in love or annoying each other) forever.