Isabel lives alone in the family home, in the provincial Dutch quiet of Overijssel. She is nearly 30, though, and unmarried, and in this 1960s backwater a woman cannot live this way indefinitely. Her younger brother Hendrik holds no claim over the house, but their older brother does: it will fall to Louis as soon as he brings home a bride. Their Uncle Karel has willed this, and since he is the one who secured the house for the family in the first place, Isa lives there only as custodian – that is to say, only on sufferance.

Isa suffers, too, under terrible loneliness. This expresses itself through a cramping kind of possessiveness. Covetous of the patterned plates and curtains that came with the house when the family acquired it, Isa insists all is tended and dusted and polished as her late mother would have wanted – and all the while she suspects the maid, Neelke, of cutting corners and pocketing teaspoons.

Yael van der Wouden does not encourage us to like her protagonist in these opening chapters. Perhaps what we feel for Isa approaches understanding – but never more than that. When they moved here, she was a child still, it was wartime, the family were fortunate to acquire such a house; Uncle Karel – always resourceful – saw to this, and Isa remains beholden. Now Isa is an orphan of sorts: tied to the house, but with no life of her own.

If we soften towards her, it’s through Hendrik. He pays his sister visits, sees the way she pinches at the back of her hand when she’s nervous, which is often. Hendrik has managed a semi-escape from the house and its obligations, living in the city with Sebastian, his French-Algerian boyfriend – a fact not properly acknowledged by the family. Early on, he tells Isa they may move to Paris. Perhaps, with Hendrik’s help and example, she can manage something similar?

Then Louis turns up with a woman in tow. We’ve already heard enough about this eldest brother to know that he falls in love easily, and out again just as fast. But Louis insists that this time it is different – and this woman certainly is. Brassy blond and in too-tight clothing, Eva is an oddity, difficult to place: an outlier in this neatly ordered middle-class world. She eyes it all with a boldness, moreover, a kind of overfamiliarity that riles. Isa sees her touching the countertops, the vases, notices the sheen of sweat on her palms; every move Eva makes seems to overstep a mark. What does she want? After Louis installs her in his mother’s old bedroom, she even tucks a snapshot of her own mother in the frame on the dressing table.

And then Louis departs, leaving Eva with Isa for the whole summer. Just the two of them in the house.

Isa steps up her counting of teaspoons, watching Eva’s every move. Soon she cannot take her eyes off her dress seams, her nape, her mouth – does Eva see this as well? “Was this always your bedroom?” Eva asks, wandering the room where Isa sleeps. All her questions come in the same vein: impertinent and strange, designed to get under Isa’s skin. When Hendrik and Sebastian pay a drunken visit, Eva dances with them in the living room, pulling Isa to join in – and the charge between the two women can no longer be denied.

The summer weeks that follow are electric, and the questions they raise are urgent. Is this passion reciprocal? Could Eva be Isa’s chance? Isa knows she can’t leave, but knows she can’t be without Eva. “‘I can’t just do whatever I want. I can’t -’ And immediately, a flash of last night came to her: her mouth slippery on Eva’s, her hand on Eva’s thigh.”

The novel gets stuck here too, though. The two women reach and grasp, have sex against door frames, on the floor, and at greater length than the narrative – up to this point – can accommodate. It just doesn’t move the story on.

The book’s third act, though, is inspired. It belongs to Eva, and with her the novel comes into its own. Even if you’ve already guessed at her own family’s wartime story, the detail revealed in this last act still has the power to kick you in the gut. If you haven’t, then to divulge more here would be to issue the mother of all spoilers. Suffice to say, the German occupation is still very recent, as are the atrocities, injustices and opportunism that flourished under their watch. The book’s powerful final act provides an already weighty emotional situation with an extra layer of historical heft.

This is an impressive debut; I already look forward to Van der Wouden’s next. She can draw characters with nuance, without fear too; she creates and sustains atmospheres deftly, and ultimately delivers a thrilling story.

• The Safekeep by Yael van der Wouden is published by Viking (£16.99). To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply.