What makes a pirate? Is it the cutlass, the distinctive tri-corner hat, or the swashbuckling disdain for authority? The pirates of W.S. Gilbert and Arthur Sullivan’s “The Pirates of Penzance”—which runs through May 2 at the Agassiz Theatre—share a radically different defining characteristic: profoundly patriotic monarchism. As they sing in the show, “With all our faults, we love our Queen.” Faults or no, the irresistible energy of the cast makes “The Pirates of Penzance” one lovable show.

The pirates’ unorthodox love for the Queen is by no means the only paradox “The Pirates of Penzance” presents; in fact, Gilbert and Sullivan seem to have delighted in irony. The plot rests on an absurdity built into the contract of Pirate Apprentice Frederic (Benjamin J. Nelson ’11), whose nurse signed him up to serve as a pirate not for 21 years but for 21 birthdays—an unfortunate choice of terms considering that Frederic was born on February 29, which means that at age 21 he’s had only five birthdays.

Despite his situation, Frederic sings of his love for a good paradox, and the Harvard-Radcliffe Gilbert & Sullivan Players (HRG&SP) seem to share that affection. And, ultimately, it’s the way the Players handle these paradoxes with earnest delight and abundant charm that makes “The Pirates of Penzance” so irresistibly enjoyable.

Most enjoyable of all are a series of three magnificently overblown introductions that build up a cast of bombastic, magnetic supporting characters: Mabel, the Major-General, and the Sergeant of Police. Mabel (Bridget P. Haile ’11), Frederic’s love interest and daughter of Major-General Stanley, bursts onto the scene with a warbling, upper-register tour-de-force that—in addition to causing Frederic to visibly swoon—immediately captures Mabel’s simple-hearted desire to impress.

Her father (Benjamin T. Morris ’09) follows suit with his hilarious introductory song, “I am the very model of a modern Major-General.” He delivers his performance with confident, prim inflection at a break-neck pace, capturing the Major-General’s character from the first sure-handed note. His assurance that he can provide “many cheerful facts about the square of the hypotenuse” provides laughs.

Considering the relative smallness of his role, the Sergeant of Police (Philipp W. Grimm ’11) is the biggest surprise of the show. Grimm’s Chaplin-esque strut and alternatively glazed and manic eyes make his Sergeant absolutely aloof but somehow loveable. He heads up a police corps that proves just as memorable as the pirates themselves.

However, these pirates do not disappoint. The Pirate King (Ilan J. Caplan ’10) has a gravitas that makes one of the show’s many exuberant refrains infectious: “But I’ll be true to the song I sing, and live and die a Pirate King.” The strength of the supporting cast sometimes threatens to overwhelm Frederic’s character—which Nelson, either purposefully or not, imbues with a sense of weakness—but that is, perhaps, the point. Frederic—who stubbornly holds to his idea of duty even in morally complex situations—is essentially a feeble character, and only his love for Mabel begins to change that.

The cast’s near-absolute success undoubtedly owes much to the show’s orchestra, conducted by music director Jesse C. Wong ’12. The overture is a highlight, tracing the course of the story in its movement from a gentle beginning to an adventurous final section, underscored by a mood of light cheer. The entire production is an outgrowth of this delightful music, and the orchestra’s vigorous playing and professional—yet relaxed—attitude make them an integral part of the show. When “Pirates” is at its best, it feels more like a musical celebration than an opera.

Other elements of the show’s design are subdued, helping the performers establish the celebratory atmosphere. The costume design of Janice J. He ’11 is all simplicity and pastels—bright, but understated. Likewise, the set pieces designed by Matthew B. Bird ’10 are blocky and cartoonish, supplementing the play’s feeling of unreality. The pirates’ boat onstage as the play begins points to this abstract, yet effective impression. These designs rightly place the focus on the performers themselves—as one particularly memorable episode in which the curtains close behind Frederic, Ruth, and the Pirate King illustrates—and those performers seldom fail to charm.

The show may not charm all equally. Gilbert and Sullivan’s humor is invested in pun and wordplay, a mixture of high-brow and slapstick that may stray too far in either direction for some. The sheer length of the play means the performers are up against the task of sustaining a frenetic pace set by the vigor of Mabel and the Major-General’s introductions. Yet, the constant introduction of new characters and the performers’ unfailing energy generally meet that challenge.

If nothing else, “Pirates” conveys that what makes a pirate is not his eye patch, but his love for the Queen. Similarly, what makes this rendition of “The Pirates of Penzance” is nothing more complex than its cast and crew’s sense of fun.