World War I gets the Masterpiece Theatre treatment in “Testament of Youth,” a lush, curiously decorous adaptation of the memoir by Vera Brittain, a young woman who became a prominent pacifist as a result of the deaths of her loved ones and the human devastation she witnessed during her service as a volunteer nurse at the front in France. In the hands of James Kent—whose background is, in fact, in British TV work, and makes his feature debut here—it emerges as the modern equivalent of a tasteful woman’s picture of the 1940s, more “Mrs. Miniver” than “Paths of Glory” (although, of course, it lacks the former’s blunt patriotism as much as the latter’s grit). For some, of course, that will be welcome news.

Alicia Vikander, who recently scored in “Ex Machina,” gets to play a full-fledged human being here as Brittain, who’s introduced early in 1914 as a headstrong, determined young woman angered by the insistence of her loving but traditional father (Dominic West)—backed by his submissive wife (Emily Watson)—that their daughter give up her dreams of attending Oxford and instead hone the skills that will make her a good marriage prospect. Vera is supported by her brother Edward (Taron Egerton), who’s about to graduate from a posh public school and go on to Oxford himself, but her self-preparation for the entrance exam appears to be wasted effort.

Edward’s bucolic summer vacation sees him and Vera frolicking in the countryside around their estate with two of his school chums—stiff, eager Victor Richardson (Colin Morgan) and handsome, athletic Roland Leighton (Kit Harington). Edward is clearly hoping for Vera and Victor to hit it off, but ultimately it’s Roland, whom she initially rebuffs, who turns out to be her soul mate—a thoughtful young man with whom she shares a love of literature and an urge to write poetry. (He also respects female strength, having a mother who herself was an accomplished author.) Her father eventually relents and allows Vera to sit for Oxford, and though she’s at first dismissed as a mere dilettante by a stern female don (Miranda Richardson), she’s accepted on grounds of her independence of mind.

Only Victor matriculates with her, however; her brother and Roland eschew university to volunteer for service in the war, which breaks out in August. Like most others, they believe that the conflict will be short and glorious, but of course it turns into the terrible slog of trench warfare that drags on for four years. Vera leaves Oxford to work as a volunteer nurse to the wounded first in England and then in France, where she’s assigned to tend to German soldiers as well as British ones, and comes to appreciate their common humanity. That experience, and the personal losses she suffers during the protracted fighting—which won’t be revealed here—lead her to become, by war’s end, a committed believer in the brotherhood of man and agitator for the peaceful resolution of international disputes.

But “Testament of Youth” is also a love story of the romance that grows between Vera and Roland, both before he departs for France, when the two get to know one another under the watchful eye of her Aunt Belle (Joanna Scanlan), who acts as their chaperone and during his service, when they exchange letters and meet during his rare, and all too short, leaves, when he briefly returns to England. Combat deepens him just as her experience does Vera, but their love endures and they become engaged, the ceremony scheduled for immediately after his completing his time in the field.

Adapter Juliette Towhidi has managed to boil down Brittain’s reminiscences into a coherent, if episodic form, in keeping with the BBC’s comfortably predictable treatment of such period material. (The Beeb dramatized it once before on film, as a 1979 television mini-series.) The ultimate feeling one gets from the script, and Kent’s practiced but unimaginative treatment of it, is of solid craftsmanship that hearkens back to an earlier age of filmmaking (there’s even a long tracking shot of battlefield wounded that’s an obvious homage to the famous one in “Gone With the Wind”). The essential problem with the approach is that it feel entirely too easy and comfortable for material of such depth; even the hospital sequences are staged to avoid being too discomforting. And the homeland scenes, from the initial pre-war halcyon days of rambling along winding roads and dips in nearby streams through the typical sequences of tearful troop departures and troubled reunions on the seaside, are so prettily done (thanks to production designer Jon Henson, art director Chris Wyatt, set decorator Robert Wischhusen-Hayes, costume designer Consolata Boyle and especially cinematographer Rob Hardy) that they seem more like paintings on a museum wall than emotional episodes. In short, the film renders Brittain’s wrenching experiences in sentimental, romanticized tones that value pictorial tastefulness over dramatic power.

Nonetheless Vikander seizes the opportunity to demonstrate her versatility, as well as the sheer charisma, the mark of a movie star, that she showed in “Ex Machina.” Harington, meanwhile, manages to exhibit the combination of virility and dewy-eyed sensitivity that the film’s portrait of lovesick Leighton needs. The rest of the cast comes across as largely functional, doing the sort of solid, professional work one expects of British second-tier players.

“Testament of Youth” is a smooth, elegantly crafted adaptation of Brittain’s account of her journey to committed pacifism; but its very conventionality and tactfulness undermine its power.