The "challenge" in Challenge of the Super Friends: United They Stand refers to the ongoing conflict between the Super Friends and the villainous Legion of Doom, who are constantly launching schemes. The title specifically refers to a 2004 DVD release that compiles four episodes from the series, including one where the Super Friends must face their origins, a time-travel plot by the Legion, and the Giants of Justice. 
 
1. Trial of the Super Friends
2. Monolith of Evil
3. The Giants of Doom
4. Secret Origins of the Super Friends