How big is the “Scooby-Doo” franchise? Big enough to get in its own way. Easily the longest-running Saturday morning cartoon to feature a talking dog and a mystery van, Scooby and friends star in two competing specials on two networks owned by the same mega-media giant. Ruh-roh, Shaggy!

The Cartoon Network offers the U.S. broadcast premiere of “Scooby-Doo and the Loch Ness Monster” (7 p.m.). The gang travels to Scotland for the famous Highland Games, hosted by Daphne’s cousin (a leggy teen in a tartan kilt) at the family castle.

Apparently the noise and commotion prove too much for the legend of the Loch, and mayhem ensues accompanied by much splashing and crashing. Eventually the shenanigans take a back seat to deduction, reason and thorough examination of empirical evidence, as Scooby’s posse gets to the bottom of Nessie’s nettlesome behavior.

• At the very least, the repeat of the 2003 special “A Scooby-Doo Halloween” (7:30 p.m., WB) spares us the predictable vulgarity of the “Grounded for Life” Halloween episode (preempted for “Doo”).

In this half-hour cartoon, the kids heads to a creepy town to spend the holidays with Velma’s relatives and celebrate “100 years of Halloween.” The holiday has been around a lot longer, but who wants to argue with a cartoon that’s been around for nearly 40 years?

• Speaking of entertainment properties that began during the Lyndon Johnson administration, “Star Trek: Enterprise” (7 p.m., UPN) welcomes a guest star from an earlier incarnation. Brent Spiner (“Star Trek: The Next Generation”) appears as the brainy but amoral Dr. Arik Soong, a scientist responsible for creating a race of genetically altered “Augments.” When his mutant critters commandeer a Klingon ship, Archer enlists the wily doctor to help track them down.

In the best tradition of “Star Trek,” this crisis inspires some discussion about the pros and cons of genetic engineering, and even a quote or two from Nietzsche.

Late night

• Richard Gere and Jason Randal appear on “Late Show with David Letterman” (10:35 p.m., CBS) … Jay Leno welcomes Paris Hilton, Orlando Jones and Anita Baker on “The Tonight Show” (10:35 p.m., NBC) … David Alan Grier and Kimberly Williams-Paisley appear on “Jimmy Kimmel Live” (11:05 p.m., ABC).
  ¢ Gisele Bundchen, Jesse James and Auf Der Maur chat on “Late Night with Conan O’Brien” (11:35 p.m., NBC) … Lil’ Wayne performs on “The Late, Late Show” (11:37 p.m., CBS).