The cartoon starts with Tom rowing on a bucket amongst some docks using a broomstick under the crescent moon (which doubles as the "C" at the beginning of the title), singing the Neapolitan ballad "Sta. Lucia" the whole way in an operatic baritone voice as the title cards are shown. As he reaches the docks, he finds Jerry rowing in a small cup and mimicking him.
Tom gets the cup, steals some tea, sugar, and a spoon from a nearby cruise ship, and covers Jerry with both before he can finish. Tom then begins to drink the Jerry-tea, but an orange cat (who is named Dupli-cat) pulls on Tom's tail, points at an empty plate, and holds his hand out as if to say, "Mine." Tom gives him the teacup, and then Dupli-cat pulls on Tom's tail again and Tom then returns the spoon. Tom then innocently sits on the pillar until he hears Dupli-cat drinking the tea, and then after a few seconds, Tom blows his top.
Tom enters the ship and sees that the teacup is sipped clean. He walks through the cruise ship, and then sees Dupli-cat running through an opening in the wall, seemingly a reflection of him. Tom continues walking back and forth, and the two cats mimic each other, as if in a mirror, in numerous ways. Tom catches on when he moves to the other side, then Dupli-cat makes a horn voice, saying "Choo Choo!" then surprised, Tom says "Woo woo?!'". Dupli-cat raises his head when Tom does not, and then tricks Dupli-cat into opening his mouth: Jerry is in it. Tom walks away and then stops with realization.
The two cats chase across a pier, and Dupli-cat holds out Jerry for Tom to take. Tom holds out his hand as he finds Dupli-cat, and then Dupli-cat stomps on a trap-door and Tom falls through it. Tom angrily climbs up the ladder and Dupli-cat closes the trap-door, knocking Tom back down into the water.
Dupli-cat then prances along the pier and Tom is shown to be doing the same on a lower layer. He snaps a loose board in the pier and it hits Dupli-cat, shunting him back into another one of the pillars. Jerry sighs with relief until Tom grabs him. Tom then runs along the pier, but fails to see another pillar and runs into it. Dupli-cat steals Jerry and ties him to his tail, and then ties Tom's fingers together around the pillar. Tom moves his fingers up and down the pillar and makes it come loose and fall on top of Dupli-cat. Dupli-cat falls through the pier and slowly sinks into the water and Tom grabs Jerry.
Tom and Dupli-cat perch on separate ships and Dupli-cat throws a bottle of champagne at his rival, knocking him out. Some of it falls on Jerry and inebriates him. The two cats then successively grab the mouse, but Jerry drifts up to an outcropping.
In an uncharacteristic display of bravado, probably due to being drunk now, Jerry (who's seemed annoyed, not afraid, as the cats fight over him) tricks both cats into poking their heads close to each other and then ties the two cats by their whiskers and tails to a dock post. Then the mouse, singing "Sta. Lucia" once again, drunkenly hiccups and then stumbles off the bubbles comes out everytime when he hiccups and some of them forming the words "THE END" which ends the cartoon.