Jinks, Myka and Pete show up at a prison in Arkansas after someone uses an artifact to melt walls and bust out two convicts named Davis and Barton, but the surprises don't stop there. Steve's ex, a handsome marshal named Liam, is on the case, and while he seems happy to see Steve, it doesn't work the other way around. Artie is hoping they all solve the case in time to be back for Claudia's 21st birthday party, which he planned down to the minute, despite everyone's pleas that he just let her get her first legal beer. While collecting Jerry Garcia's black light from the stacks to heighten the décor of the party, Artie knocks over a rubber ball that went bouncing off of Mozart's clock and is now causing Artie to hear – loudly – all of the master's works.
Myka and Liam follow the evidence to Davis's girlfriend, the owner of the artifact, a bronze jar from Pompeii that spouts lava. Barton trapped her in her car while he used the jar to kill Davis, then fled to a hangout for his old gang looking for someone named Chris. Jinks does everything he can to keep their search for the artifact separate from Liam's search for the fugitive, for official and personal reasons. But when Liam gets a hit on Barton and goes on the hunt, Jinks has to follow him – if Liam doesn't know what he's up against, everything could go wrong, and Liam could wind up dead. In a tense car ride, the two exes finally have the breakup talk that never happened, but they're far from a resolution when they arrive at a remote garage where gang members are initiating a new recruit into the fold. It's Barton's son, and they're forcing him to shoot a security guard.
Myka and Pete learn that Chris is Christina, the mother of Barton's child. Chris tells them that Barton's going to use the artifact to drag his son from the grips of his old gang, and they arrive at the same garage as Jinks and Liam just as Barton pulls out the jar and the gang members pull their guns. When Liam orders them to drop their weapons, chaos ensues and Barton drops the jar, covering the entire garage floor in a torrent of lava. The thugs are swallowed by the lava while Barton and his son find a ladder to cling to, but Barton got shot in the arm and can't climb very high. Myka and Pete surf from car to car until they reach the jar, and Pete lowers Myka down to where she can close the jar and bag it. The hot lava instantly cools as Barton loses his strength, but is saved by the snag and bag. He must now return to prison, but his son is safe and has a newfound relationship with his dad. And Jinks and Liam came closer to each other during their brush with death, and though they don't reunite, they do have a really good goodbye.
Artie goes deaf thanks to bumping into Mozart's clock. He and Claudia discover Mozart's clock has a missing part, a bust that was recently sold to a hoarder. The buyer's nephew sniffs out that the bust is more valuable than supposed and tries to abscond with it, but Claudia fells the guy while the now nearly-deaf Artie delivers a knockout punch and bags the bust. His hearing returns and they celebrate their teamwork.
They close the case just in time to celebrate Claudia's birthday! Turns out everybody should've minded their business when Artie was planning the party, because he called in a favor and got one of Claudia's idols – Cherie Currie from The Runaways – to play at the local bar! Claudia does a duet of "Cherry Bomb" with the legend, officially making it the best birthday ever.
On a rainy day at the Warehouse, Pete accidentally triggers an artifact that transforms the team into two-dimensional diorama versions of themselves, and returns the Warehouse to its original, unstable design.
Who knew Claudia had those moves?! OK, so maybe they're artifact-induced, but we're so in love with the spangles, we don't care!