Marvin is an employee at Irish pub Shenanigans that Shawn and Gus talk to when Shawn discovers that a recent string of robbery victims all had the same four-leaf clover stamp on their wrist. Although annoyed at Shawn's mockery of his outfit and faux accent, Marvin tells the duo that the stamp is used during the pub's speed-dating event. Shawn and Gus go undercover together with Lassiter and Juliet at one such event in an attempt to find the robber, and notice that Marvin is always in the vicinity of the event, for example to refill the popcorn.
Marvin is later taken into the police station when he is seen loitering outside the home of one of the speed daters and a huge stack of speed-dating questionnaires is found in his apartment. Marvin explains that all he did was take the questionnaires out of the trash to increase his own chances of getting a date. Although he does not match all the clues at the crime scene, Shawn did spot a green smudge that means Marvin is most likely involved with the robberies and asks Chief Vick to let him go so that he can lead them to his partner.
Vick grants Shawn's request and the gang go back for another round of speed-dating. Shawn notices that Glenda, a supposedly single woman he speed-dated himself the first time, is actually married from a tan line on her ring finger. When Shawn spots the same tan line on Marvin's finger, he knows the two are married and outs them to the entire crowd. He explains that Glenda would always ask her speed dates bank security questions such as their mother's maiden name, their favorite color, and the names of any childhood pets they had while making it look like regular small talk. After getting this information, she and Marvin would follow the target and rob them of their credit cards. This was made easier by the intoxicating GHB-laced popcorn that Marvin provided the speed daters, but it also killed one of their victims when the drug interfered with a pre-existing heart condition. The crowd are outraged and the two crooks are arrested.