The title of the episode is a bit misleading, since on this occasion Eddie is the one being double-crossed rather than the one doing the double-crossing. Eddie has acquired a steady girlfriend, the attractive Caroline Shuster. Full of cocky sophistication gained from watching Tony Curtis movies, Eddie boasts to Wally about having Caroline "wrapped around his little finger" and makes a great show of calling her syrupy endearments amid the milkshakes at the malt shop. However, Wally later overhears Caroline mocking Eddie behind his back and understands at once that Eddie's girl is a two-faced liar. As any good friend would do, Wally warns Eddie that he is being played for a fool, but Eddie becomes belligerent and a rift quickly develops between the two friends.
Interestingly the rift is not repaired by the end of the episode, though a chink of hope is opened as a repentant Eddie approaches Beaver and asks him for advice on how to make up with Wally. Here we get a rare glimpse of the sincere, vulnerable Eddie.
This is one of the stand-out episodes of the Fourth Season, though an atypical one for LITB in that not everything is tidied up by the end. Eddie Haskell and Wally Cleaver are starkly contrasted here: Eddie is a conceited blowhard and Wally is modest, self-effacing, responsible and mature. The writing and direction, in many subtle ways, drive home the point that it's the latter type of person who not only wins the day but gets the girl. The episode also features a slice-of-life view of 50's-era teenage social life with the scenes at the malt shop.
A telling and poignant moment comes when Wally is about to break the bad news to Eddie in his room. In the midst of bragging about his great relationship with Caroline, Eddie idly picks up a portrait of Wally and Beaver sitting on Wally's dresser and starts making fun of it and the brotherly relationship it represents: "Man, this is really corny. You and your grubby little brother." At this point Wally has had enough and shoots back: "Look Eddie, why don't you just turn it off for a minute and sit down, huh?" We, as the audience, know that Eddie sadly lacks the warm family life enjoyed by the Cleavers, and his bluster is a cover for that fact.
Why was Wally friends with Eddie in the first place? It's a mystery; but undoubtedly Eddie was the better for Wally's steadying influence.