"Threshold" (9 p.m., CBS): Those freaky aliens just keep wreaking havoc in the chilling freshman

"Threshold" (9 p.m., CBS): Those freaky aliens just keep wreaking havoc in the chilling freshman drama "Threshold." The saga continues with a new episode that focuses on a disc jockey who becomes infected and winds up incorporating the mysterious audio signal into her music. Sounds like big trouble for Carla Gugino and company.
"Alchemy" (8 p.m., ABC Family): A professor's experiment pits a humanlike computer against a real human to see which will be more attractive to a woman. Complications ensue, of course. This new movie stars Tom Cavanagh, Sarah Chalke, James Barbour, Michael Ian Black and Illeana Douglas.
"C-SPAN Call-In Marathon" (8 p.m., C-SPAN): C-SPAN marks a quarter-century of opening its phone lines to viewers with a 25-hour call-in marathon. The program will include highlights of past shows, along with guests and various hosts, live from C-SPAN's studios. Want to know where to call? You'll have to tune in: The number will change throughout the broadcast.