If based only on its central —and sole — on-screen performance, Nightingale is a somber and harrowing film. David Oyelowo, this year’s near Oscar nominee for his towering performance as MLK in Selma, commits himself wholly and completely to this one-man bottle film with ease and plenty of range. Never for a second is he afraid to explore the damage, isolation and sometimes humor of his character, Peter Snowden.

If director Elliott Lester could evoke more resonance beyond his actor’s commitment, however, this undoubtedly would be a powerhouse feature. As it is, however, it’s a commendably paced look at guilt, grief and the deranged, but one meant to be introspective but mostly only feels surface-level.

Snowden, in the wake of family tragedy, is a man isolated inside both his house and consensus, as Lester’s tale focuses squarely on the character’s continued mental instability as the weeks go on. Held by the promise that his old war buddy Edward will come to his quarters, Nightingale is a movie which evolves narrative and quality-wise as it continues — one done intentionally, the other not, of course.

The editing becomes less jagged and the writing more flourished, but Nightingale also feels more comfortable with its gimmick as it continues along. The need to keep the story encapsulated no further than Snowden’s front lawn often seems forced up until around the second half. The character is finally fully drawn, and with that, the need to keep him in the house seems as natural as can be.

The entire production, as expected from its single character and setting, has a staged feel. Thus, it’s surprising to realize it’s not only an original screenplay but also one from a first-time writer, Frederick Mensch. It’s a finely tuned piece of work — and certainly it should be if it gathered the attention of executive producer Brad Pitt — one that not only centers itself on its character but also rightfully peels the layers with dutiful attention. Mensch makes sure the audience knows only select details about Peter’s life and mental state in a given moment, so as to make the character feel more developed as Nightingale progresses. And as we’re trapped inside the walls of both Peter’s house and his own mind, he and Lester are wise in chipping the details ever precisely through the feature’s mere 82 minute running time.

That said, Nightingale is a disappointingly unsubtle film. For all the immediacy brought through its single location and almost uncomfortably tight cinematography by Pieter Vermeer, Peter still feels distant. Even when his emotional arc becomes fully realized, there’s something oddly aloof about his perspective, to the point where it’s hard to find him as compelling and haunting as he’s desired to be. Sure, he’s deranged and unstable, but there’s little here thematically which hasn’t been told before. There’s also little particularly shocking or innovative about how it’s told, and its decent execution makes it tolerable but it never hits you as hard as it should.

Additionally, for as many testimonials Peter gives for his vlog via his iPhone or laptop, it’s never particularly straightforward what Nightingale’s primary intentions are with his character. Is he meant to be more a reflection on PTSD, one place in the world in relation to God or a commentary on our super self-centered Internet culture and need to make the personal public? Perhaps it’s meant to be a combination of the three, as the second comes with heavy visual cues but never directly influences the plot or main character’s actions. That Mensch wants the viewer to take away from Peter whatever they can is clear, but without one clear focus it’s hard to decipher what Oyelowo’s character means to the filmmakers.

Regardless, as an intelligently painted examination on mental instability in the wake of tragedy, sorrow and often self-pity, Nightingale nevertheless becomes a fairly provoking character study. This is all a testament to Oyelowo. It’s a remorseful, wholehearted performance, one never afraid to linger or meditate even when Lester oddly doesn’t focus on his work in favor of bland visual metaphors. Fierce and tender, what’s most astounding about the overall film is how it makes the protagonist a sympathetic character, even though his problems at least partially result from his own actions. As simply a reminder of what great things the actor can do, this new HBO film succeeds. But the promise that more can be explored and committed here remains caged in its own, housebound seclusion.