There are requests you should not hear out to the end. I heard Nevzad's out, one March evening in the Two Lions café on the Obala, while the Miljacka carried dirty snow down from Trebević. We had known each other since second grade in Hrasno. We failed our first driving test together, we got our first jobs together, he as a structural engineer in a design office, I as a translator at an agency in Marijin Dvor. At his wedding to Ilvana, two years earlier, I had been his best man. Ilvana worked at a pharmacy in Grbavica, laughed quietly, and read books with a pencil in her hand. Nevzad used to say she was the only woman who had never asked him how much he earned.

That evening he turned his glass around and said he was happy, and that this was exactly the problem. "I load bridges until I see where they give way," he said. "Only then do I know they are sound. Ilvana is faithful because nobody has ever seriously tried to win her. That is not faithfulness, that is a lack of opportunity." He wanted me to be that opportunity. Three months, he said, a real courtship, messages, flowers, everything. If she resisted, he would sleep peacefully for the rest of his life. If she did not, better to know now than in twenty years. I laughed, thinking he was joking. He was not joking.

"A woman is not a bridge, Nevzad," I said. "A bridge does not hold it against you that you loaded it." I talked for a long time and, as it seemed to me then, cleverly: that you do not test what you have by breaking it, that I would lose both her and him, and he would lose both of us. He listened patiently, the way he listens to investors who do not understand the calculations. Then he said that if I would not do it, he would ask Izudin from the office, who the whole firm knew never let a single waitress go by. That was when I agreed. I told myself I was agreeing only to protect her, and that I would fake all of it.

For the first weeks I was an honest liar. Nevzad invented reasons to leave: a meeting in Doboj, an inspection on a bridge over the Bosna. Before the first time, he gave me the spare key to the apartment, on a key ring with a little plastic City Hall, "in case Ilvana needs anything." I would come over, Ilvana would make coffee, and we talked about her customers who wanted antibiotics for every sneeze and about my translations of dishwasher manuals. Afterward I reported to Nevzad that I had told her she was beautiful, that I had sent her a poem, that she had frowned and changed the subject. He was proud of her and, it seems to me, a little disappointed.

One Sunday he invited me to lunch and, while Ilvana was in the kitchen, showed me her phone. He knew her passcode. There was not a single poem in our messages. There were three recipes for apple pie and one picture of a cat on the neighbors' balcony. "You lied to me," he said, and he was not angry but hurt, which was worse. "I thought you were my friend." That evening I promised him I would do it properly. The next Monday he left for three weeks in Doboj, on a real inspection, and I stayed in Sarajevo with the key in my pocket.

I do not know exactly when I stopped pretending. Maybe the day we walked down the Velika aleja to Vrelo Bosne because Ilvana wanted to see the swans, and came back by tram, soaked, laughing at something I could not repeat today. Maybe the night I wrote to her about the way she held a coffee cup with two fingers, as if it were heavy for her. She answered only the next day, with one word: "Don't." I kept writing to her. I wrote because I did not know what else to do with everything I had learned about her.

Later I found out that she had called Nevzad that week. She told him his best man was sending her messages he should not be sending and asked him to come home. Nevzad answered that she was exaggerating, that Vlatko was like a brother, that she should stay calm and he would be back in two weeks. I do not know what she felt when she ended the call. I only know that two days later, on the balcony above the stadium in Grbavica, she kissed me first, and that the stadium below us was empty and wet with rain. After that we closed the doors behind us, and I stopped counting the lies.

When Nevzad came back, I told him Ilvana was made of stone. That she had refused the flowers, deleted the messages and threatened to tell him everything. He hugged me in the parking lot outside the office and said he would never forget what I had done for him. For the rest of the spring the three of us lived in that contentment. He was happy because he had a faithful wife, and the two of us because he believed it. Ilvana's younger sister Senida, a pharmacy student, was already living with them by then, in the small room next to the bathroom, and on Fridays she went to their parents in Zenica.

One morning in June, around six, I was walking past their building and saw a young man come out of the entrance and pull up his hood. I did not know him. Something turned over in me, stupid and fast: if she could cheat on Nevzad with me, why not on me with someone else. Still angry, I called Nevzad and told him Ilvana had given in. "On Friday, tell her you are going to Doboj," I said. "Stay in Senida's room and listen for yourself." I ended the call, and within five minutes I knew I had done the worst thing of my life.

I went to Ilvana at the pharmacy and told her everything. She looked at me like a man she was seeing for the first time. The young man in the hood was Senida's boyfriend from Kakanj, she said, and he came over when everyone was asleep, because Nevzad would have thrown them both out of the apartment. Then she fell silent, lined up the boxes of medicine on the shelf, and told me what we would do on Friday. She spoke calmly, as if she were reading out dosage instructions. That was when I understood that I had fallen in love with a woman whom even I did not know.

On Friday evening I rang the bell, even though I had the key. Ilvana opened the door and did not let me past the hallway. She said, loudly enough to be heard through the closed door of the small room, that I had been sending her messages for two months, that it was shameful for a best man to do this to the man who had given him the key to his home, and that she had kept quiet only because she did not want to break a friendship that meant more to Nevzad than anything. Then she took the key out of my hand, stepped out onto the landing and dropped it down the stairwell, between the railings. I heard it clatter three floors below. She slammed the door behind me. Nevzad, she told me later, came out of the room in tears and knelt in front of her in the hallway.

That night he sent me a message: "Thank you, brother. Now I know." For the next few months he was the best husband in Grbavica. He took Ilvana to Jahorina, invited me for ćevapi on Sundays, and raised his glass to the best man who had freed him from doubt. We did not stop. We stopped only in October, when at half past five in the morning Nevzad found the young man from Kakanj in the hallway and threw him out barefoot into the street. Senida, in tears and furious, told her brother-in-law he had better ask his own wife where she spent her Tuesdays. That same night Ilvana packed a bag and came to me in Ciglane.

Nevzad came the next day. He did not shout. He just stood in the doorway in his work jacket with the office logo. He took out of his pocket the key with the plastic City Hall, a little bent. "I found it that night under the stairs," he said. "I wanted to give it back to you for your birthday. As a present." He set it down on the doormat and left. Ilvana stayed with me for eleven days. On the twelfth morning there was a note on the table saying she was going to her mother in Zenica and that I should not look for her. I did not look for her. Neither she nor Nevzad ever contacted me again.

Now I live in Ljubljana and translate manuals for machines that know on their own when to stop. The apartment in Grbavica was sold three years ago, Senida wrote me that in the only holiday card I ever received from anyone in that family. The key is still in the drawer of my desk, under my passport. It opens a door behind which live people I have never seen, and sometimes, when I cannot sleep, I wonder whether they have changed the lock.