Sitting on a cot that barely fit into his dark nook, the prisoner, Maxim Kucheryavy, was obviously hungry and in pain.
A Ukrainian volunteer who wanted to fight pro-Russian insurgents, Mr. Kucheryavy said he had been shot in both legs at point-blank range while unarmed and already in detention.
Bandaged and limping, all Mr. Kucheryavy wanted now was to go home, just a three-hour drive from here.
His jailer's home is much farther away.
Mamiyev is from North Ossetia, a Caucasus republic that is part of Russia.
He had never set foot in Ukraine until three months ago.
The owner of a private-security business, Mamiyev has a kickboxing background and combat experience in Russia's war with Georgia in 2008.
He says it wasn't money that enticed him to fight Ukrainians here, but a complicated mix of attachments, including to God, to Russian President Vladimir Putin and to the Russian nation, even though he is an ethnic Ossetian.
Mamiyev is part of a chaotic insurgent movement that, with Moscow's help, has brought to east Ukraine the kind of warfare—featuring assorted militias, artillery, tanks and trenches—that calls to mind World War II and the 1990s Balkan conflicts.
Initially caught flat-footed by the separatists, Kiev has since turned the tide of the war through heavy use of the regular military, combined with newly formed volunteer battalions of the kind Mr. Kucheryavy, the Donetsk prisoner, was hoping to join.
As a result, the rebels have been steadily losing ground and are now concentrated in their largest remaining stronghold, the city of Donetsk.
If the pace of the Ukrainian offensive continues, the entity known as the Donetsk People's Republic may well be entering its last days, though insurgents vow to fight back here with everything they've got.
The war has summoned out of the woodwork an eclectic mix of mostly local men looking for a lofty goal to leaven their daily existence—and finding it in separatism.
Russian citizens are in top leadership positions, both on the battlefield and in the separatist government.
There are also many rank-and-file gunmen from Russia.
Moscow says it doesn't back the separatists and that Russian citizens participating in the conflict are volunteers.
Roman Lyagin, the minister of social affairs in the separatist government, said he recently received millions of dollars worth of Ukrainian hryvna, shipped as bundles of bills on wooden pallets.
The source: Crimea, the southern peninsula which since being annexed by Moscow in March has accumulated vast reserves of the Ukrainian currency as it shifts to the Russian ruble.
He said the money would go toward social payments and cash compensation for the families of dead insurgents, who are to get around $60,000 apiece.
These aren't just days of unremitting warfare for the insurgents.
Though many of Donetsk's restaurants have shut down, a 24-7 joint called Banana flourishes hosting a clientele consisting mostly of rebels and their friends or associates, many armed and in uniform.
They exchange hugs, lean weapons against tables and conduct conspiratorial chats over glasses of tea and shots of vodka.
Some bring dates.
In a city where regular people are afraid to drive because cars—especially fancy ones—have a tendency to get stolen, curbside parking at Banana can feature an Audi SUV pulling up late at night with the kind of reckless screeching and disregard for speed-bumps that often accompanies the operation of a vehicle the driver doesn't own.
While some rebels party at the Banana, Mr. Kucheryavy, the Ukrainian prisoner, continues to languish in a basement across town.
A floppy-haired 25-year-old with no military experience beyond the occasional harmless game with replica firearms, Mr. Kucheryavy said he volunteered to fight the rebels after they shot down a Ukrainian troop-transport plane in June, killing all 49 soldiers on board, including a close childhood friend of Mr. Kucheryavy.
Insurgents bragged widely about that attack, in contrast to their public silence over the downing of Malaysia Airlines Flight 17 on July 17 that killed all 298 people on board.
Kiev and Washington blame the insurgents for shooting down the Boeing 777, perhaps mistaking it for a Ukrainian military aircraft.
Rebel leaders say they had nothing to do with it.
The tragedy enraged the world but barely registered on the Ukrainian battlefield, where the two sides continue to pound each other with escalating ferocity even as Dutch-led investigators collect body parts at the crash site nearby.
One night in late July, Mr. Kucheryavy packed a light backpack and set off by train on a covert journey to link up with a Ukrainian volunteer battalion operating deep inside insurgent-held territory alongside regular Kiev troops.
"Ukraine's independence depends on you," he wrote on his social-media page.
He posted a photo of a railway station by night with a cryptic caption below: "If you are planning to go somewhere…absolutely do not postpone it!! There may not be another opportunity."
His journey ended quickly.
Rebels detained him en route, well before he could reach his battalion or pick up a weapon.
They beat him with a baseball bat, turning the length of his arms into a purple-blue mess.
Then, one interrogator pulled out a pistol and shot at Mr. Kucheryavy's right leg.
The bullet pierced the soft tissue of both thighs, and Mr. Kucheryavy collapsed.
Mr. Kucheryavy's mother, Svetlana, a hospital nurse in the city of Dnipropetrovsk, adopted him when he was 10 days old, and he is her only child.
She has diabetes, and recently had some of her toes amputated.
She walks with difficulty.
The two lived together, and Ms. Kucheryavy only learned of her son's plans when he left for the railway station.
Ever since his disappearance, Ms. Kucheryavy has been frantically calling Ukrainian officials, and praying for him.
Though beaten and wounded, Mr. Kucheryavy fared better than others held in the same makeshift jail in the town of Horlovka, he said.
Until recently it was the domain of a widely feared pro-Russian commander nicknamed Demon.
Lying beside Mr. Kucheryavy on the cold floor, he said, were two men with bags tied over their heads and kneecaps shattered by bullets.
With Horlovka under attack by Ukrainian forces, Mr. Kucheryavy was soon transferred from Demon's custody to another rebel militia in Donetsk.
He was thrown into another basement, where he said his treatment improved.
There is a reason for that.
Mamiyev, his Ossetian jailer, wants to exchange him for an insurgent held by Ukrainian forces.
"Have they fed you yet?" he asked Mr. Kucheryavy.
Upon Hearing that he hadn't eaten, Mamiyev ordered an underling to scrape together a meal.
The prisoner drew quickly on a cadged cigarette, squinting at the unfamiliar sunlight as he limped to a bench during a rare break outdoors.
A rebel nurse has treated and bandaged his wounds.
Mamiyev allowed his prisoner to make a quick phone call to his mother.
"Everything is OK here," Mr. Kucheryavy told her.
"I just want to get home terribly."
After he hung up, he admitted that captivity is wearing him down mentally.
"I'm starting to drift," the prisoner said.
Mamiyev said he had boarded a boat in the spring to be smuggled across the Sea of Azov into Ukraine.
In and then made his way to Donetsk.
Here, he joined a local rebel battalion with an Ossetian component.
There were Chechens fighting too, though most have left, he said.
He has fought, gone back to Russia for medical treatment, and returned.
Besides a mortar fragment still lodged in his leg and two wasp bites from a swarm that has taken up residence inside a tree on the rebel base, Mamiyev has also acquired a Porsche here, a gift from fellow insurgents who stole it somewhere in Donetsk.
Mamiyev said he doesn't want the unsolicited present.
His commitment to the fight at hand is so single-minded and fierce, he said, that he'd be willing to follow the biblical tale and sacrifice his son—not for God, but for the Russian state.
Source: The Wall Street Journal