The BBC television movie “Elizabeth Is Missing” — a stand-alone episode of “Masterpiece” on PBS this Sunday — contains Glenda Jackson’s first screen performance since 1992. That certainly merits attention — Jackson, now 84, is one of the most technically accomplished and ferociously intelligent actresses of our time. Did it merit the rapturous British reviews on its release in 2019 and perhaps inevitable awards, including a BAFTA and an international Emmy, that she received for it? Not really, but it isn’t Jackson’s fault.

You can see the appeal to Jackson of “Elizabeth Is Missing,” which was adapted by the actress and writer Andrea Gibb from a mystery novel by Emma Healey. The central character, Maud, who is moving from forgetfulness into dementia, is onscreen virtually the entire time, whether in the present or as her teenage self (played by Liv Hill) in a parallel story line set 70 years ago. The progress of the film largely takes place through Jackson’s twofold embodiment of Maud’s decline and of her stubborn, often angry battle to delay and deny it.

The story puts Maud in a situation full of dramatic promise: her best friend, Elizabeth, has suddenly disappeared, and Maud is determined to find her despite the inconvenient fact that she can’t convince anyone that Elizabeth is actually gone. Scrawling notes to herself about Elizabeth’s glasses and some suspiciously broken vases, Maud carries on her investigation in fits and starts, picking it up again whenever she remembers that Elizabeth is missing.

It’s a great setup for a straightforward mystery, but “Elizabeth Is Missing” is more complicated than that, and while you can’t hold that ambition against it, you might wish that you were watching something simpler. Maud’s search for Elizabeth is woven together with the disappearance of Maud’s married older sister in 1950. Events in the present and past continually mix in Maud’s mind, her memories triggered by objects or phrases in ways that are artful and a little too self-conscious.

The mystery-novel structure of the story turns out to be both a feint and a reality, something that becomes predictable fairly early on and is disappointing in the final result. We’re supposed to be getting a deeper satisfaction from the detailed depiction of Maud and her affliction, and the neatly arranged thematic resonance between the two story lines, revolving around what it really means to be missing.

But despite the efforts of the talented director Aisling Walsh (“Maudie”), who gives the film a welcome restraint and clarity, “Elizabeth Is Missing” doesn’t hit the mark — the screenplay is too fussy and tricky, and the resolution to the twin mysteries, with its mixed notes of heroism and resignation, isn’t convincing. (Walsh’s final image, a long shot of Maud crossing a street alone in mourning clothes, has a power lacking in the rest of the film.)

But as you could expect, it contains a mostly faultless performance by Jackson, one that’s certainly worth 87 minutes of your viewing time. (It might also remind you that despite Jackson’s stature, and some high points like “Sunday Bloody Sunday” and “The Return of the Soldier,” her screen résumé isn’t all that distinguished.)

She doesn’t play for our sympathy — she leans into the character’s frustration and irascibility, making it clear how difficult she is to deal with. And she communicates Maud’s flickering moods and perceptions precisely and indelibly, in the way she briskly taps a notecard when Maud makes a connection or in a quick, shattering moment when she silently screams with frustration at a restaurant, conscious of not making (too much of) a scene. Maud may not come fully alive in the script, but there’s nothing missing in Jackson’s portrayal.