Viewers may find themselves giggling at “Lights Out” (Warner Bros). But it won’t be because this feeble horror film has scared them silly.

Injecting humor into the haunted house scenario, screenwriter Eric Heisserer and first-time director David F. Sandberg undercut the expected terror. The result is predictable and inspires few chills.

Martin (Gabriel Bateman) has a better reason than most 10-year-olds for not being able to get to sleep. His demented mother, Sophie (Maria Bello), is camped out nightly in her dark bedroom with her best friend, a feral creature named Diana (Alicia Vela-Bailey). Zombie-like Diana is repelled by light, which is why Martin sleeps with a flashlight.

The backstory reveals that Sophie and Diana met as children, when both were committed to a mental institution. Diana contracted a rare skin disease which rendered her hideous and, ultimately, invisible. It was also fatal — or so the doctors thought.

Fast forward 20 years. Sophie is twice married, off her meds, and acting very strangely when the sun goes down.

As the body count starts to rise, Sophie’s estranged daughter, Rebecca (Teresa Palmer), enters the picture. She left home years ago, but is now determined to rescue her brother.

At a brisk 81 minutes, “Lights Out” doesn’t unduly tax viewers’ patience. Unfortunately, the film’s ending is not only unexpected and shocking, it’s also morally unacceptable.

Were the context any less remote from real life, the movie would have to be considered unsuitable for all. As it is, the otherworldly situation within which the climactic misdeed is committed diminishes its likely influence, meaning that at least a few well-grounded grownups may choose to witness it.

The film contains occasional bloody violence and scary imagery, a suicide, implied nonmarital sexual activity, drug use and some crude language. (L, PG-13)