In Memoriam

The season premiere of “Riverdale” will serve as a tribute to cast member Luke Perry, who died in March. The actor played the father of K.J. Apa’s Archie. His former “90210” co-star Shannen Doherty will make a guest appearance. Creator Roberto Aguirre-Sacasa has said the hour is “probably the most important episode of ‘Riverdale’ we’ll do this year, if not ever.”

7 p.m., WUCW, Ch. 23

Building a mystery

The series premiere of “Nancy Drew” wastes no time in establishing that the title character is no longer the sleuth you grew up with. In the opening moments, she’s finishing up a quick sexual encounter, the first of many signs that our sleuth has more on her mind than just solving neighborhood crimes. This version ends up relying more on “Veronica Mars” than author Carolyn Keene’s original book version. Pop culture fans will get a kick out of recognizing the town psychic. She’s played by Pamela Sue Martin, who starred in the comparatively tamer 1970s series, “Nancy Drew Mysteries.”

8 p.m., WUCW, Ch. 23

Top of the food chain

“Nature’s” presentation of “The Serengeti Rules” is about 30 minutes shorter than the theatrical version, but it doesn’t take much time to get the general idea: Keynote species, like starfish, sea otters and wildebeests, are absolutely critical to the ecosystem. The film makes a pass at presenting the scientists behind this discovery as heroes, but you’ll be oohing too much over those otters to notice.

7 p.m., TPT, Ch. 2

Neal Justin