A sold-out crowd of sharp-dressed music lovers was treated to a second big dance last night as My Morning Jacket took the stage to open their two-night, “Under the Sea”-themed residence at Athens, Georgia’s famed 40 Watt Club.

Fans — decked in everything from vintage suits and gowns to homemade robot, whale and penguin costumes — turned out early to find streamers and silky scarves cascading from the ceiling and bars draped in fish nets and plastic sea creatures, all bathed in the glow of red and blue Christmas lights. Balloons floated overhead in the back, where a makeshift photo lab offered commemorative prom pictures, complete with a cardboard sleeve bearing the My Morning Jacket insignia. Meanwhile, opening act Follow the Train — themselves dressed as crabs, seahorses and starfish — warmed up the crowd with a “couples only” rendition of the 1955 classic “Earth Angel.”

After much anticipation and a hearty welcome, My Morning Jacket appeared donning silver plastic Elvis wigs and over-the-top suits that could have come straight from the wardrobe for Dumb and Dumber. Wasting no time with chit-chat, the band dove straight into a cover of The Dells’ doo-wop classic “Oh, What a Night” and blazed through a charged set of reverb-drenched rock & roll that had fists pumping and hair shaking from wall to wall. As Jim James’ haunting voice echoed through the club and a psychedelic light show shimmered from behind the stage, it seemed pretty clear that this was prom as it never was but always should’ve been.

The stellar hour-and-a-half set included staples spanning the band’s eight-year career, including favorites like “Off the Record,” “Wonderful Man,” “Gideon,” “Run Through,” and “Golden,” which induced a collective sing-along from the dance-happy crowd below. For the most part, James and company kept the songs relatively short and coherent, but occasionally broke into extended jams that illustrate why My Morning Jacket has earned a reputation as a must-see live act.

During the encore — which included a cover of King Harvest’s “Dancing in the Moonlight — James excitedly announced the night’s Prom King and Queen, who joined the band onstage for their official dance. As the happy coupled swayed to the rhythm, James topped off the evening with a trannie-friendly cover of Eric Clapton’s “Wonderful Tonight”: “I put on your make up / I put on your bra / I slip on your panties / I slip on your tights / And I say Oooo! / I look wonderful tonight.”

“To the class of 2007, the class of 1957 and every class in between,” he called to the crowd, “You could have celebrated your prom at a lot of places tonight, but you chose to spend it here. We thank you.”