Detective Comics #837 2007

This 2007 issue of Detective Comics showcases a unique team-up between the reformed Riddler and Harley Quinn. Bruce Wayne hires the Riddler to find a missing Wayne Enterprises employee who stole a metahuman serum. Their investigation leads to the Athenian Women's Help Shelter, where Harley Quinn aids in recovering the stolen goods. This story is notable as a "Countdown" tie-in, revealing a surprising connection between Athena and Darkseid, and highlighting the Riddler's new role as a private investigator.