(Be forewarned. Parentheses lie ahead.)
Cercis siliquastrum, the Judas tree, is thought in folklore to be the kind of tree that Judas hanged himself after betraying Christ. While a number of linguists see this is a folk etymology based on a French phrase meaning "tree of Judea" the legend persists, not that the two ideas are mutually exclusive. Whether or not Judas Iscariot hanged himself from this tree, or any tree, or in fact was really the betrayer of Jesus, the name has a certain Biblical resonance, and there's a slight aura of Christian allegory to this episode. In that it feels like an Ellery Queen novel, some of which took on an allegorical role after Manfred B. Lee's conversion from Judaism to Episcopal Christianity.
(Jonathan Creek has also taken a title from the Judas tree. I can't review it because I've only seen the early episodes with Jonathan and Maddie)
The story here touches on the Christian - albeit not exclusively Christian, of course - themes of sin, justice, guilt, atonement, and redemption. This does not prevent it from being jam-packed with assholes.
After some opening business with the body, we cut to Ellery and Richard Queen at home. The former is under the sink, attempting to fix a leak and only making the problem worse. Velie arrives to inform his boss that there's been a murder in Brooklyn Heights. The Inspector leaves his son to deal with the mess from the sink and goes to the scene of the crime. While Brooklyn Heights is part of New York City the house is effectively a country manor. Victim George Sherman has been found hanging in his garden, a wreath of flowers from the Judas tree on his head. Ellery arrives driving in a helpless and reckless manner that, combined with the ongoing plumbing fiasco, makes a strong case for his sacrifice of basic skill for awesome training. Thereupon comes the first twist: as Inspector Queen informs Ellery, George Sherman was stabbed to death before he was hanged. Oh, and what's the awesome training? Well for one, Ellery takes a quick scan of the victim's study and from a look at the cups on a shelf tells his father to look for a left-handed killer.
As for the suspects, boy do we have some doozies. His former business partner is smug about both gaining full control of their business at a cheap price and being out of town when the murder was committed. The firm's council is the kind of lawyer who makes obscene amounts of money basically just to keep his mouth shut. The widow and the victim's doctor, aside from being lovers, are also just horrible people on every level you could name and some you probably couldn't.
(Cast notes on this motley crew. In order we have: Jack Kruschen, who played Jack Lemmon's neighbor and conscience Dr. Dreyfuss in The Apartment; Dana Andrews, full-fledged movie star of The Best Years of Our Lives and Laura, by this time in a period of genteel obsolescence; Diana Muldaur, sister of Geoff, former sister-in-law of Maria, and best known for her later roles on Star Trek: The Next Generation and LA Law; and George Maharis, one of the handsome lads of Route 66.)
Outside the household we find another two intriguing figures. One is Steven Yang, who knew Sherman in China during the war and had reasons to want revenge against him. Before he contacts Ellery everyone just calls him "that Oriental." This sounds jarring to contemporary ears, since we'd just use the perhaps overly broad "Asian." Of course the venom comes not just from cultural prejudice, but from the widespread belief that he's a murderer. When we meet him he turns out to be a raconteur, a gifted amateur painter, and a lover of ascots. He also claims to have found a broken man, and one who we know was already dying, so why bother killing him?
Then there's Father Devlin, a missionary who also knew Sherman overseas. The leader of an international charity, Devlin wears glasses that make him look like a less gleeful Dr. Strangelove, can't make eye contact with either Queen, and makes clumsy attempts to guide his own interrogation. Today his shiftiness would be a sign that he was crazy or had a sexual proclivity that you sometimes hear about priests having. In the more innocent time this was made it signified that he wasn't really a priest at all.
Perhaps coincidentally there was a real priest in England named Ronald Knox who not only wrote mysteries but also set down ten rules - implicitly parallel to the Ten Commandments - for writing whodunits. The unraveling of this episode's mystery requires a violation of one of Fr. Ronald Knox's rules. You could say it breaks two rules, but his injunction against including a "Chinaman" in a story was aimed at supernatural Eastern menaces like Fu Manchu. Mundane suspects who happen to be from China are a different story.
(BTW, Archie Goodwin, the "Watson figure in the Nero Wolfe series, is not at all stupider than the average reader. Neither is John Watson, especially when you factor in his being a surgeon. So while these are reasonable guidelines I'm not sure the best writers ever followed them, ahem, religiously.)
At the end comes the revelation. Ellery is mild and his father is somewhat courtly, but don't be fooled. They are there to ferret out the guilty. Even if this happens to be a case of "it's not the crime, it's the cover-up" some cleansing has been done.