There are some secrets in the backyard pool

As is the case with supernatural thrillers, “Night Swimming” is a rapid decline; It’s convincingly acted and quite effective in its first half, disorganized and dazed in its second half. Blumhouse Productions’ latest release works from a premise we’ll call “the haunted house next door.” The house is fine. But the pool in the backyard mod.

The last family to realize this is the Wallers. They’ve moved around a lot. Father Ray (Wyatt Russell), a promising major league third baseman, has gone through the usual trades but has been sidelined in recent months due to symptoms of multiple sclerosis and associated rehab. The replacements also included Ray’s school administrator wife, Eve (Kerry Condon, a superb Oscar nominee for “The Banshees of Inisherin'”); their naturally gifted athlete daughter Izzy (Amélie Hoeferle); and his socially isolated younger brother Elliott (Gavin Warren), a boy who lives in the shadow of his successful, somewhat ruthless sister.

A new house somewhere in the Twin Cities bodes well at first, but the pool of evil begins to hint early and often. A mysterious, battery-powered toy boat appears on the surface of the pool and moves on its own. Unseen or barely visible creatures (human or formerly so) from the beyond draw adults and children alike to the water. Elliott hears the sad, ghostly voice of a girl trapped behind one of the vents. He is the one who sets up the narrative in the prologue set in 1992.

This all works for a while, in part because the Wallers’ tensions and anxieties are portrayed more realistically than the typical supernatural thriller family unit. Condon is the ringer, making every scene, if not believable, then at least realistic. The idea for “Night Swim,” Bryce McGuire’s second feature-length directorial debut, from a script he co-wrote with Rod Blackhurst, is based on the ominous underwater world beneath the spring-fed pool.

At a key point in the second half, Eve visits one of the house’s previous owners. Jodi Long is very good in this role, but this is where “Swimming in the Night” begins to paddle up rivers of exposition, exposition, and mild to moderate anger for the audience. Still, I like the duality of the pool’s nature: a healing natural spring for some, a watery hell for others.

“Night Swim” comes from a word cunning 2014 short Directed by Blackhurst and McGuire, the film is just under three minutes long, minus the end credits. Apples and oranges, I guess, but the short does more work in terms of atmosphere and rhythmic tricks than the full-length version. Still: These filmmakers have a past and future of evocative horror. Marco Polo’s traditional game of billiards has long awaited its frightening show. (Did I hijack an earlier thriller to capitalize on this?) And throughout “Night Swim,” Mark Korven’s (“The Witch”) wonderfully uneasy score evokes sound waves that the film can’t always match with the images.

“Night Swimming” — 2.5 stars (out of 4)

MPA rating: PG-13 (for terror, some violent content and language)

Running time: 1:38

How to watch: Premieres in theaters January 4

Michael Phillips is a Tribune critic.

mjphillips@chicagotribune.com

excitement @phillipstribune