Entertainment

Has-beens back in action

John Lithgow is old enough to play Neil Patrick Harris’ dad on HIMYM. (CBS)

Monday is apparently National Whatever Happened to…. Day, as evidenced by the guest stars populating TV throughout the evening:

“Castle” (Monday, 10 p.m., ABC) If you’re not a regular viewer of the daytime dramas, you might not realize that “L.A. Law’s” Corbin Bernsen had a steady paying gig last year on his mother Jeanne Cooper’s show, “The Young and the Restless.” He pops up in primetime with a few melodramatic compatriots, including “Where Are They Now” nominee Jane Seymour, as part of this week’s soap-themed murder storyline.

“The Event” (Monday, 9 p.m., NBC) Gabrielle Carteris, better known as Andrea of “Beverly Hills, 90210.1,” guest stars as one of the Sleepers whom Thomas (Clifton Collins Jr.) calls so he can disclose the contents of the message he received.

“Hawaii Five-0” (Monday, 10 p.m., CBS) Larisa Oleynik was Nickelodeon’s attempt to replace the popular Clarissa (Melissa Joan Hart) on “The Secret World of Alex Mack.” She’s all grown up now and playing a CIA agent who’s interested in tracking down the killer of McGarrett’s (Alex O’Loughlin) parents.

“Harry’s Law” (Monday, 10 p.m., NBC) Remember “Queer Eye for the Straight Guy”? Only vaguely? Yeah, well the little guy who didn’t really have a discernable skill, Jai Rodriguez, plays a drag queen that Harry & Co. must defend.

“How I Met Your Mother” (Monday, 8 p.m., CBS) Mama doesn’t want to ruin this comedy’s dramatic turn this year as Barney (Neil Patrick Harris) made the mature decision to find his dad, but John Lithgow is guest starring. You do the math. — Tiffany Connors

Whoa, daddy: If your tween suddenly develops a tummy ache Monday morning, consider this: There’s a “Pretty Little Liars” marathon from 11 a.m. to 8 p.m. in the run-up to the season finale of this ABC Family teen serial, as the girls finally put an end to the “A” mystery.