"AMERICAN EXPERIENCE" (9 P.M., PBS): GET OUT YOUR LOVE BEADS AND BE SURE TO WEAR SOME FLOWERS IN YOUR HAIR! PBS' "AMERICAN EXPERIENCE" TAKES YOU ON A TRIP TO 1967 FOR "SUMMER OF LOVE," A GROOVY, MIND-

"American Experience" (9 p.m., PBS): Get out your love beads and be sure to wear some flowers in your hair! PBS' "American Experience" takes you on a trip to 1967 for "Summer of Love," a groovy, mind-expanding flashback to a cultural phenomenon full of lofty ideals, but also broken dreams. The place was San Francisco's Haight-Ashbury district, where, that summer, thousands of young people from across the country came together to this sudden hippie mecca. But even before they got there, this happening was already on the wane -- and the swollen population jamming sidewalks, parks, soup kitchens and health clinics only hastened its demise. It had begun mere months before as a nonstop celebration of free music, free love, free drugs and "cosmic oneness." It was a revolution that sprung up in response to 1950s-bred comformism and the rampant bloodshed of the Vietnam War. "Turn on, tune in, drop out!" druggy guru Timothy Leary exhorted a crowd of 20,000 at San Francisco's Golden Gate Park in January 1967, the "be-in" that led to that summer's overwhelming invasion. Filmmakers Gail Dolgin and Vicente Franco examine the social and cultural forces that sparked the largest migration of young people in America's history. They hear from a diverse group of people who lived through the Summer of Love (including actor Peter Coyote, who sounds like he would gladly do it all again). And they trace the indelible impact those few months has had on the nation ever since.
"Cities of the Underworld" (9 p.m., History Channel): The secret worlds of abandoned train lines, tunnels, tombs and other hidden structures are spotlighted in this 13-week series that shows the foundations of the world's greatest cities. Hosted by Eric Geller and Don Wildman, the programs follow architects, engineers and explorers who delve into subterranean secrets of cities. The premiere features Edinburgh, Scotland; subsequent episodes spotlight such cities as Berlin, Paris, Rome and New York. Highlights include a bunker once used by Winston Churchill, an ancient Roman firehouse and a secret World War II hospital.
"Heroes" (9 p.m., NBC): It's time to get serious about this save-the-world stuff. "Heroes" makes a much-welcome return from its hiatus with the first of five new episodes, which culminate with the season finale. The bad news is that evil Sylar is still on a rampage and he's about to get into it with Peter.
"A Life Interrupted" (9 p.m., Lifetime): Inspired by real-life events, a new Lifetime movie, "A Life Interrupted," tells the story of Debbie Smith, a suburban housewife and mother who was dragged from her kitchen in broad daylight and raped in the woods behind her home, while her police-officer husband was asleep upstairs. After suffering through the rape-kit test, Debbie waited for her assailant to be found. But her wait seemed endless -- six long years of fear and paranoia -- before her rapist was finally identified thanks to a chance DNA test. Determined to help women in similar circumstances, Debbie agreed to work with a forensic scientist to lobby Congress on the importance of nationwide DNA testing. Evolving from victim to advocate, she worked tirelessly with lawmakers to pass the bill ensuring all usable forensic evidence is processed quickly and efficiently so that sexual predators are brought to justice. Lea Thompson ("Caroline in the City") stars as Debbie Smith in the film.
"Diners, Drive-Ins & amp; Dives" (10 p.m., Food Network): Celebrity chef Guy Fieri (host of "Guy's Big Bite") moves to prime time with the debut of his new series "Diners, Drive-Ins & amp; Dives." From establishments that have stayed the same since opening day to young owners who are modernizing classic ideas for a new generation, Fieri will look far and wide to showcase the most unique, fun and delicious locations he can find. The half-hour series will premiere tonight.
"The Real Wedding Crashers" (10 p.m., NBC): This new reality series uses a "crash team" of five improvisational performers who go undercover as wedding staffers and guests to liven up a wedding. These crashers play various characters including the pastor, the wedding planner, a waiter, a long-lost friend and even an exterminator, with the bride and groom in on the fun as they assist the crashers in pulling pranks on the guests. At the end of each show, the crashers own up to their pranks. Steve Byrne ("Comedy Central Presents"), Ben Gleib ("The Gleib Show"), Desi Lydic ("Pepper Dennis"), Catherine Reitman ("My Super Ex-Girlfriend") and Gareth Reynolds ("The Room") star as the crashers. Inspired by the hit feature film "Wedding Crashers," this real-life version is produced by Ashton Kutcher and Jason Goldberg. It premieres tonight.