The Zoo, which has a score by Sullivan and a libretto by Bolton Rowe (pseudonym of B.C. Stephenson), was first performed on June 5, 1875, a few weeks after Trial By Jury opened; it was produced as an afterpiece to Gilbert's farce Tom Cobb. Like Trial by Jury, The Zoo is a one-act piece without spoken dialogue.

The major characters are: Aesculapius Carboy, an impecunious apothecary; Laetitia, whom Carboy wants to marry; Mr. Grinder, Laetitia's father; Eliza Smith, who runs the Refreshment Stall of the London Zoological Gardens; and Thomas Brown, Eliza's suitor, a man with a secret.

As the curtain opens, some "Ladies and Gentlemen of the British Public" (a.k.a. the chorus) are visiting the zoo's Bear Pit and Refreshment Stall, and (of course) singing. They finally notice that a young man, Aesculapius Carboy, is trying to hang himself on the veranda of the Refreshment Stall. They have no objection to his doing so, but want to know why: "Is it your wife?" Carboy explains that the father of Laetitia Grinder, the woman he loves, has rejected him because he is a mere apothecary. Now a further problem has arisen: he simultaneous sent Laetitia a dose of peppermint (the lovers communicate through prescriptions) and her father a medication not to be taken internally--and apparently the labels got mixed up.

Eliza Smith, who is in charge of the Refreshment Stall, orders Carboy to desist (after all, it would probably hurt business if his corpse were dangling from her stall). Thomas Brown, who has been wooing Eliza, suspects that Carboy is a rival, but she reassures him.

Laetitia arrives in search of Carboy; it seems that her sister sent the message about the mix-up of labels as a joke. The two sing a duet of rapturous reunion, as Eliza and Thomas Brown sing their own duet in counterpoint--a patter-song about all the refreshments Thomas has been consuming while handing around Eliza's stall. Then Thomas collapses; Carboy, as an apothecary, insists that he must not be crowded or moved, so the chorus crown around Thomas and try to move him, while offering a multitude of conflicting second opinions. Finally Thomas comes to and announces weakly: "It was the last bun." Carboy sends Eliza off to get a prescription for him. Then, continuing his medical examination of Thomas, he discovers the Order of the Garter beneath his coat and diagnoses that "he's a peer in disguise." Thomas, recovering, admits this and explains that he has come here incognito "in search of virtue" and found it in Eliza. He goes off to change his costume.

Laetitia's father, Mr Grinder, now appears, in search of "my wicked daughter" and carboy. He rebukes them, calling her "Heartless undutiful child" and him "Vilest Compounder of potions." They (and the chorus) try to appeal to Grinder's finer feelings, but unfortunately he doesn't seem to have any. So Carboy again resolves on suicide and sings "Fetch me a rope!" The chorus, ever helpful, provides him with one. Carboy sings a touching farewell to Laetitia, and uses the rope to lower himself into the Bear Pit.

At this dramatic moment, Thomas Brown--actually the Duke of Islington--returns in all his ducal splendor and reveals his True Identity to Eliza, asking her to marry him. She is reluctant to leave "the beasts I loved so well," distraught at the thought that without her and her refreshments, the grizzly bears and other animals might not be fed. But Thomas, truly a Sensitive Male before his time, reassures her: he has bought all the animals, apparently as a wedding present, and the baboon, raccoon, and so on will accompany them. As he tenderly sings:

Every morn, at early dawn, the gentle armadillo,
Or rattlesnake, when you awake, you'll find upon your pillow.

Eliza is touched by his thoughtfulness. As they sing of their happiness (accompanied by the chorus), Carboy's voice is heard from the Bear Pit. ("Great Heavens! I had forgotten, " remarks Laetitia.) Carboy reemerges from the Bear Pit, and the chorus is annoyed at his arousing their sympathy and then not dying. He explains that the bears seem to have been moved, but, instead, he will "try the lion's den." Thomas, however, moved by his devotion, has come to a financial arrangement with Grinder to obtain his permission for Laetitia's marriage to Carboy, and gives the two of them "double the amount"--ten thousand a year. So all prepare for a future of joy unbounded (with wealth surrounded) as the curtain falls.