Mak Plain was a Muun species member, specifically a male banker. He served the InterGalactic Banking Clan, operating from both Coruscant and Scipio, frequently acting as a go-between for the Bank and the Galactic Republic.
During the time of the Clone Wars, Mak Plain was employed by the InterGalactic Banking Clan, sometimes found in his Coruscant office. Following the failed Confederate–Republic peace initiative, the Galactic Senate started to move forward with the Republic military enhancement bill. It was at this point that Plain made public a new interest rate of twenty-five percent on loans to the Republic, in addition to the Banking Clan's funding of three million battle droids for the Confederacy of Independent Systems. This announcement greatly angered many Senators, including Senator Padmé Amidala.
When Senator Amidala launched an investigation into corruption within the Banking Clan, Mak Plain greeted her and her companions upon their arrival to the planet Scipio. During a heated discussion concerning the appointment of Senator Rush Clovis as the new head of the InterGalatic Banking Clan after the corruption scandal involving the Core Five ruling council, Mak Plain stated that the Muuns supported Clovis for the role.
The character of Mak Plain was developed for the Star Wars: The Clone Wars animated television series, with Stephen Stanton providing his voice. Plain's debut was in the episode "Pursuit of Peace," which was broadcast on December 3, 2010.