Lily Tomlin, left, and Jane Fonda in “Moving On.” Aaron Epstein/Roadside Attractions

In “Moving On,” “American Pie” writer-director Paul Weitz reunites with Lily Tomlin, his acerbic leading lady in the film “Grandma,” for an attempt to re-create the delicious fusion of dark comedy and drama he forged with her in that 2015 abortion-themed story.

Here, the sober subject of his new film is sexual assault, in an icky and not especially amusing plot that pairs Tomlin with Jane Fonda as old pals who reunite at the funeral of a friend to confront the dead woman’s husband (Malcolm McDowell) over a decades-old encounter.

Fusing a smattering of old-folks jokes – references to fading memory, poor eyesight, catheterization, Viagra, arthritis and other ailments – with a narrative of buried trauma is a risky gamble. But Weitz and his game cast do try.

Malcolm McDowell in “Moving On.” Glen Wilson/Roadside Attractions Glen Wilson/Roadside Attractions

Fonda’s Claire is a twice-divorced octogenarian who flies from Ohio to California after the death of her friend Joyce, announcing to Joyce’s widower, Howard (McDowell), immediately upon arrival at the funeral that she intends to kill him. Enlisting Tomlin’s Evelyn – who lives in a retirement facility, where she has access to a resident’s flare gun – as an accessory, Claire prepares to dispatch Howard. When she proposes a meeting with her potential victim, and Howard suggests they get together at a local park in the company of Howard’s daughter (Sarah Burns) and three grandkids, Claire replies, incongruously, “Perfect!”

That’s just one example of several actions that don’t add up, even in what is ostensibly a silly comedy. Tomlin, tart-tongued as ever, does her best to leaven the proceedings: There’s a throwaway subplot involving Evelyn’s mentorship of a young child (Marcel Nahapetian) who likes to wear her high heels and earrings, but it doesn’t go anywhere, and the charming, light touch of Tomlin and Nahapetian’s scenes together feels out of place in the larger context. A rekindled romance between Claire and her first husband (Richard Roundtree) – whose marriage is revealed to have disintegrated around the time of the old assault – is also a dead end.

All that aside, Tomlin and Fonda have a pretty nice, unforced chemistry together, cultivated over years of working together on “Grace and Frankie” and other projects (see “80 for Brady”). McDowell, on the other hand, is a more one-dimensional villain. His dark fate in the film is handled in a way that’s meant to be laughable – and is, but not in the way that’s intended.

There’s lots of moving on referenced in “Moving On”: death, divorce, gender transition and other momentous life events, including the letting go that Claire is, at least initially, so unable to achieve. But there’s something strangely static and unmoving at its center. It’s an emotionally stagnant affair, whether it’s going for laughter or tears.