Hints of seasonal change abound as several networks show off their fall wares. The cast of “How I Met Your Mother” hosts a “CBS Fall Preview” (7:30 p.m., CBS), a showcase of upcoming shows. Over on Fox, viewers will get a chance to catch the “director’s cut” of “Glee” (7:58 p.m., Fox), the high-school musical satire that Fox previewed last May. Original episodes of “Glee” begin next Wednesday. Over on selected PBS stations, viewers can get an advance peek at the new Ken Burns documentary series “The National Parks” (8:30 p.m., PBS, check local listings).

All of this talk of the “new” season hearkens back to a TV era when the networks were the only game in town and viewers waited for new shows to return with season premieres designed to coincide with the release of the new car models. This hasn’t been the case for decades. The most talked-about and written-about series of the summer are probably “Mad Men” on AMC, which just started its third season, and “True Blood,” which will end its second season on HBO the Sunday after next.

• In addition to looking ahead, Fox invites viewers to glance back at the best of “So You Think You Can Dance” (7 p.m., Fox), recapping the top performances from the past five seasons. “America’s Got Talent” (7 p.m., NBC) also asks viewers to look back — to last night — and then picks four more acts (8 p.m.) to move on to the finals.

• Shows like “The Rachel Zoe Project” put so much emphasis on the glamour of the Oscar that it’s almost impossible to imagine there was a time when some people were too frightened, or too proud, to accept their statuettes. “American Masters” (7 p.m., PBS, check local listings) profiles Dalton Trumbo, an acclaimed and successful Hollywood screenwriter (“Kitty Foyle,” “Thirty Seconds Over Tokyo,” “A Guy Named Joe”) who, along with other writers, was blacklisted and sent to prison for refusing to cooperate with a House committee investigating communist influence in Hollywood.

The film recounts Trumbo’s efforts to survive as a writer and a family man while cut off from his livelihood and betrayed by those he once considered friends and colleagues. Under a series of pseudonyms, he continued to pound out screenplays. He wrote the story for “Roman Holiday,” one of the most beloved American films of all time. In 1956, the Oscar for Trumbo’s screenplay for “The Brave One” went to a “Robert Reich.” Only there was no Robert Reich, and no one claimed to be Robert Reich, and his Oscar went unclaimed.

Trumbo would return to credited screenwriting with “Spartacus.” The film’s famous “I am Spartacus” scene, when thousands of Roman slaves lay claim to a doomed man’s fate, clearly mirrors the Robert Reich experience, when no one would pick up a scapegoat’s Oscar.

The best reason to watch this profile is the dramatic readings (by Brian Dennehy, Joan Allen, Paul Giamatti, Michael Douglas, Nathan Lane and others) of Trumbo’s extraordinary letters to friends, moneylenders, a phone-company executive and others. A letter of advice to his son is breathtaking. Filled with anger, eloquence, passion and poetry, they reveal Trumbo’s rare facility with both language and logic. They are more powerfully reasoned than any mere drama or fiction. You can easily see why all of these accomplished actors couldn’t wait to perform them.