“Life After Beth,” starring Aubrey Plaza and Dane DeHaan, seems like it would play out in typical indie horror-comedy fashion, with a deft blend of farce and drama, good performances, and solid writing. But unlike “Zombieland” or “John Dies at the End,” the film’s plot caves in by the third act, and all the audience is really left with is a heap of uneven tone, flat humor, and unanswered questions.

The film begins with the funeral of Zach’s girlfriend, the titular Beth. This is where the problems begin. Instead of being introduced to the couple before her death, there is nothing. We have no sympathy for either of their situations. As Zach broods for the first 15 minutes of the film, the audience simply does not care, also due in part to DeHaan’s surprisingly shallow performance. Depth of character is absent in Zach as DeHaan seems to only be able to muster up three modes of emotion: awkward sadness, awkward happiness, and really awkward anger.

Beth, played by the usually entertaining Aubrey Plaza, mysteriously rises from the grave and returns home. She slowly begins her descent into blatant zombie-dom. At first, Plaza flaunts her usual array of comedic talents (in particular her trademark deadpan delivery) that have suited her well in other films such as “Safety Not Guaranteed” and “The To- Do List,” but as the second half of the movie begins, and Beth’s undead instincts rise to the surface, out comes a side of Plaza we’ve never really seen before. The best word to describe the performance that follows would be grating, as Beth spends the rest of the film screaming, twitching, and trudging around in one of the strangest zombie performances I’ve ever seen. Normally those traits would be synonymous with the average undead portrayal, but director Jeff Baena makes the odd decision to have Beth snap back to her normal self in the middle of one of her zombie rage sessions, creating an uncomfortable bit of juxtaposition that doesn’t work nearly as well as it could have.

For a film that definitely emphasizes comedy over horror, there are not that many laugh-out-loud jokes in “Life After Beth.” The moments that do stand out involve Beth’s parents, played by John C. Reilly and Molly Shannon, and Zach’s parents, played by Paul Reiser and Cheryl Hines. Reilly’s character is a humorously overprotective hack, and Shannon plays Beth’s wispy mother perfectly. Reiser factors in fine as the straight man, and Hines completely nails the role of suburbanite mom with a snappy attention span. Nearly every time these seasoned actors were on screen, they had something funny to say or do. There are also a few lines from an undead grandpa towards the end of the film that warrant a few chuckles.

The main reason why “Life After Beth” fails as a film however is its complete lack of staying power. The concept rides out its welcome far too early, leaving the rest of the movie feeling like an unnecessary chore. The quality of everything, including the acting, the writing, the comedy, and (strangely) the directing goes to hell in the third act. It’s this strange tonal shift accompanied with the aforementioned inexplicable turn of events (why are the dead returning to the world of the living? Why and how did Beth spark this bizarre chain reaction?) that really send the film off the rails. There is an attempt to bring it back to reality with an out-of-place ending that I won’t spoil here, but needless to say it comes out of nowhere and harbors no character development whatsoever.

The film is a huge missed opportunity, a story that could be great if handled by competent writers and different leads. Some enjoyment is to be had in the parental figures’ antics as well as the use of practical zombie makeup, but the rest of the movie is a lifeless affair.

“Life After Beth” is showing at The Union Theater on Saturday, October 4 at 9 p.m.