What to Watch Monday: Witness Beware the Slenderman

Your daily guide to the most interesting stuff happening on TV and streaming. All times Eastern.

PICK OF THE NIGHT
Beware the Slenderman
HBO, 10 p.m.
This compelling documentary concerns two Wisconsin girls, Morgan Geyser and Anissa Weier, who were accused of plotting to kill a classmate and almost succeeded when they allegedly stabbed Payton "Bella" Leutner 19 times on May 31, 2014. Geyser and Weier were just 12 years old at the time, but what really fueled media interest was their obsession with Slenderman, a faceless ghoul about which countless internet short stories have been written.

MIDSEASON PREMIERE
Supergirl
The CW, 8 p.m.
Melissa Benoist is always ready to take on villains both real and fictional. <iframe src="https://art19.com/shows/ews-what-to-watch/episodes/e08e9cb3-39e3-4126-85e9-7413e09dded4/embed?theme=light-gray-blue" width="300" height="150" scrolling="no" frameborder="0" class="" allowfullscreen="" resize="0" replace_attributes="1" name=""></iframe>

MIDSEASON PREMIERE
Scorpion
CBS, 10 p.m.
These Scorpion plot lines are getting so ridiculous that I'm starting to think the writers are just trolling us: Tonight, the gang must save a man who's "teetering on the edge of a massive sink hole that threatens to destroy the Los Angeles water supply forever."