Now this is a treat. Even though the show is technically between seasons at the moment, ‘Psych’ made an off-schedule appearance on the USA Network Sunday night to bring us a special two-hour musical extravaganza, a perfectly-timed bit of silliness just as so many other series are closing up shop for the year.

Officially, ‘Psych: The Musical’ has been designated as the final two episodes of Season 7, which last aired back in May, rather than the beginning of Season 8, which will start in January. I’ll be honest that I haven’t necessarily watched every episode of the series, and I’m a little confused about where this falls in the continuity. Shawn makes a couple references to Juliet being unaware that he’s not really psychic, yet as I recall, she found out the truth in the middle of last season and broke up with him for several episodes. Is the musical supposed to take place before that, or was this just sloppy writing? Honestly, it doesn’t really matter. ‘Psych’ is hardly a serial drama that relies heavily on continuity. Taken as a standalone, the musical is delightful.

Why a musical? Why not? Lots of other shows have done it, from ‘Buffy the Vampire Slayer’ to ‘How I Met Your Mother’ to ‘Fringe’ – some to better results than others. ‘Psych’ is goofy enough to work something like this into its concept. Unlike most of those others, this episode doesn’t make much of a pretense of explaining why the characters are suddenly singing and dancing. They just are, because that’s the kind of show this is.

The plot concerns the escape of a mental patient named Zachary Zander, who goes by “Z” (Broadway star Anthony Rapp, from ‘Rent‘). Some years earlier, Z was a playwright convicted of murdering a theater critic whose scathing review sunk his musical production of the Jack the Ripper story. Since he’s been locked away, his original producer, director and most of the cast have reunited to relaunch the show under a new title without him, claiming to be working from a completely fresh script. After his escape, the producer and lead actress wind up dead, and it looks like Z is on a kill spree to exact revenge for the betrayal.

Following a brief encounter with the fugitive, Shawn believes that Z is innocent, even though all the evidence points to his guilt. As usual, no one else buys the story, especially not the ever-skeptical Lassiter. As Shawn and Gus work to find the real killer, they seek the help of Z’s only friend from the mental institution – gleeful serial killer Yang (Ally Sheedy), who claims to be reformed. Unfortunately, she takes advantage of their trust and also escapes, yet continues to Skype Shawn with oblique clues to help him.

Naturally, Yang becomes a suspect in the murders. But so do the theater owner (Barry Bostwick, who gets in at least one ‘Rocky Horror’ reference) and the lead actor. When a masked assailant attacks Shawn on the theater stage, Yang sacrifices herself to save him, her final bid for redemption. As she ascends to heaven, Yang sees Mary Lightly (Jimmi Simpson) at the pearly gates. He tells her that she probably won’t get in, but “I know a guy who knows a guy,” and he’ll see what he can do.

Eventually, it’s revealed that the play’s director is the real killer. He murdered the critic and framed Z. His allegedly all-new script for the play is in fact Z’s first draft, before it was watered down in crummy rewrites that the director himself requested.

In the final scene, we see Shawn and Gus visiting the mental institution, pitching this whole story as their idea for a stage musical to Z. Did any of these events really happen (Shawn insists so), or were they all a fantasy? The hell if I know.

Does the musical episode work? Absolutely. All of the songs are original (one even makes a dig at the ‘Grey’s Anatomy’ musical for just recycling lame pop songs) and hilariously written. Surprisingly, most of the cast here can actually sing. Even James Roday is pretty good. Timothy Omundson has a lovely rich baritone voice. Maggie Lawson is probably the weakest link, but she’s not really that bad, and isn’t required to sing much.

About to head into its eighth season, ‘Psych’ has been kind of erratic in quality the last few years, with episodes that vary from truly inspired to desperately strained. This one finds the show in top form. Everything here clicks, from the cast chemistry to the dialogue to the funny bits of randomness that clutter the background of most of the musical numbers. In one of its best running gags, Gus keeps trying to get himself cast in the Jack the Ripper musical as a new character he’s invented called Jamaican Inspector-mon. His end-credits moment of triumph is a riot.